REPORT No. 354

AIRCRAFT WOODS:

AND

TEEIR PROPERTIES, SELECTION,

CHARACTERISTICS

By L. J. MARKWARDT

Forest Products Lqboratmy

416s0-31+1 407

https://ntrs.nasa.gov/search.jsp?R=19930091423 2020-05-19T13:32:17+00:00Z

CONTENTS

Summary_______________------------------------------------------------------------------------------------Induction ------------------------ ------------------------------------------------------------------Strengthvaluesfor aircraftdesign---- --------------------------------------------------------------------------Otherpropertiesneededin aircraftdwign-------- _______________________________________________________________

TensiIestress-------------- __________________________________________________________________________Torsionalproperties_____________

----------------------------------------------------------------------- ------

Formf~tirs ------------------------------------------------------------------------------------------------Adjustmentof TabIeI vdu--------_____--_-__-_-- ------------------------------------------------------------

hloisturecontent--------------------------------- --------------------------------------------------------V*bfity -----------------------------------------------------------------------------------------------Dumtionofstmm-------- ______________________________________________________________________________

SeIectionofmaterid----------------- ------------------------------------------------------------------------Primaryfactarsin SeIectingwood--------------------------------------------------------------------------

Speciiiogravity-strengthTddiORE___----- -----------------------------------------------------------

MinimumSpecfiogravityqtiement ------------------------ -_—-_ —-------- —---------------Permissibledefects--------------------------------------------------------------------------------Prohibiteddti~ti ________________________________________________________________________________

Secondaryfactorsin selectingwood-________________________________________________________________________Localityof growth-------- -------------------------------------------------------------------Rateof growth----------------------------------------------------------------------------Positionin tree-. --------------------------------------------------------------------- .-—..------Hearhroodand=pWd ------------------------------------------------------------------

Toughness-testmethodof selectingwood---------------------- — ----------- —--- —------- —----- —Suitabilityof wciw---------------------------------------------------------------:-----------

W’oodsnowcommonfn *aft Service------- --------------------------------------------------------------WhiteMh- ----------------------------------------------------------------------------------------Balsa---------------------------------------------------------------------------------------Basswood---___ -—_--____————------------------------------------------------------------------Yellowbhch------------------------------------------------------------------------------------------SIahoguy-------------------------------------------------------------------------------------------:~-:ple ------------------------------------------------------------------------------------------

-------------------------------------------------------------------------------------------Wkdtepine-- --------------- ,_________________________________________________________________________YeIIow~plar ---------------------------------------------------------------------------------------Red, Sitka,andwhitespmce---------------------------------------------------------------------------Blackwtiut --------------------------------------------------------------------------------------

Requirementsin manufacture.- ---------------------------------------------------------------------------Mechanicalpropertiesof natives~iw ---------------------------------------------------------------------Detaileddiscuastonof sWiw -----------------------------------------------------------------------------

Hardmods(broad-Ieawxl W*)----------------------------------------------------------------------Softwoods(coniferousspecies)-------------- ----------------------------------------------------------

Concltions-------------------------------------------------------------------------------------------------Refe-cm --------------------------- ---------------------------------------------------------------------Biblio~aphy----_----:--- -----------------------------------------------------------------------------------

469

Psga

47147147147247247247347347347547547547647647s47s4s34s44344354354s64s54374s74374s74874874S7

467’

4374s748743743743743s4s9490496499500500

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.

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REPORT No. 354

AIRCRAFT WOODS: THEIR PROPERTIES, SELECTION, ANI) CHARACTERISTICS —

By L. J. MAEKWAEDT I

SU3131~RY

Wood has been one of the pioneer materials in air-craft conidruction. It8 salient qualdies-a high ratioof 8trength to weight; lightne88, affording read@ thesize of member required to reeid twisting and lateralbuckling; ease of trianufacture;facility of repair u<th-out specialized equipment and without highly 8killed&or; and adaptability to emalkale productiOn—hare always permitted it to ~erre useftiy. Althougha kck of uniformity in the quality of wood ie perhap~ themost important factor now militating again8t its con-tinued u8e in preserdday guuritily production, theexisting detailed knowledge of the propertie8 and thecauee8 of rariaiion in them, determined at the Forest%oduct8 Zuboratory and eubmitted to the NationalAdrisoqj Commiltee for Aeranautica for publication,makes ii poseible to selectaircraft materialwith a.seuranceand place8 deeign on a re.hble basis.

Strength ralue8 of rariou.s woode for aircraft designfor a 16 per cent moisture condition of material and a$ee.cond duration of 8tre88are presenled, and also adiecu88ionof the rariouefactor8 afecting the dues. Tletoughnewted method of gelecting wood is d&us8ed, anda table of acceptance due8 for 8ez’erd 8pecie8i8 gken.

l%% report presents,further, information on the prop-erties of rariou8 other natire 8pecie8 of wood comparedwith spruce, and diecu88e8the characteri8iic8of a con-siderable number of themfrom the 8tandpoint of theirpo88ibleapplication i; aircraft manufacture @ 8upple-ment the wood~ that are now most commonly used,

INTRODUCTION

Engineering structures are commody designed withthe me of safe working stresses. The method isSimple$ makes for safety, and gives satisfactoryresults when the members are subject to simpletension, compression, or bending. ‘iWth aircraft, onthe other hand, the elements of torsion, compression,and bending are often combined in the same member.

1Seder IhKI.IIeer,Forest Products IAbomtory, Forest &rifq U. S. Depdmmtof A@nlture. Mdntdned at Mdeorq Wh., h wpmutfon * the Unkadtyof wlx@DSIn.

This makes it desirabIe to design on ultimate stressessince in combined Ioading the stress is not proportionedto the loacL

Briefly, the usual procedure in aircraft design isfit to deterroine the load for horizontal @ght con-ditions, and then to estimate the maximum probableload in terms of the load for horizontal flight conditions.A factor of safety (usually 2 in the United States) isthen appIied to the maximum probabIe load, thusarriving at the design load. For exampIe, if five timesthe Ioad for horkontaI %igM conditions were decidedon as the maximum probable load, and H the factor ofsafety is 2, the load factor would be 10. The designerkeeps the stresses for the maximum probable loadtithin the eIastic Iimit of the material. Hence thefactor of safety provides some rexme strength to takecare of occasiomd owrstxes-sing, slight fluctuations inthe quality of the material and workmanship, and errorin the estimate of loads.

STRENGTHVALUESFOR MRCRAFTDESIGN

‘l?able 1 presenta strength data on various woods foruse in aircraft design. The valuea are based on amoisture content of 15 per cmt and a duration of stressof three seconds. The minimum acceptable and theaverage specific gravities are included, as well as theweight per cubic foot of matdd at 15 per cent mois-ture content. These design values have been adoptedby the United States &my Air service, the Bureau ofAeronautics of the United States Navy Department,and the United States Department of Commerce. Thestress values listed apply only ta materhd that meetsthe mMnmm requirements for speci6c gravity and thelimitation of defects; these requirement vrU be setforth Iatar.

The des&n vahes of Table I, for native specks, area result of a comprehensive series of standard t&amade at the Forest Products Laboratory; so far 164native species have been included. The identificationof species, the method of samphg, and the testingmethods followed very closely the standard procedureof the American Standards Association and the Ameri-can Society for Testing MateriaIs. CReferenca%I and 8.)

# 471

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472 REPORT NATIONAL ADVISORYCOMMITTEEFOR AERONAUTICS

TABLEL-Mwr@h ualuee oj rmrious woods, bawd on 16 per cant moisture confeti, for we in aircrqft bign—._ .

Spwlflc gravitybaaed on vol-

ume and wefghiwhen oven drv

Wnkage fromrem tooven-d&eo#i!7ib&

aionswhenm

Statfo bending ‘-Ida;loadylu

,4Wnehbatttema-heff1~11~

refght ~at 18

3%moie-tnramt.ent

Lbs.m-m.J’f.88

41

z4486

Rg

628444

46!2aw

M22

242282842a64262727

%Sefw0[WOO*nym< and Mdankal1“”-”--1-

—.Ffbw

atehs.J$t1

mdll-breoftwl

&odrrfurf elastic

Ity fi

<%~8&

wLm*s&

1:2401,m1,6601,m1,m1,5XI

1,4KI1,$3)1,4W

1,02)m

1,621I,mI, no11m1,1401,WIL040L 810L w

PorktoerlrlmMfnf-

mum~;d

0.46

:%

.59

.48

.89

.52

.4s

.71

.42

.46

.m

Aval-ege rsn-

Sntfdhdfel

%7cm6..0Lu6.6L87.08.78.94.8h2

...-.48&4L8

L64.06.2

8.8.21

4626&e&o

2;

:;L1

iK6-Ku.mcuk{~

IL 2

Ii!16.2lL 7

:.!

~. ~8.0

M

am:E.60.C8.68.48MJ

.79

.47,61.67

.69

.48

.60

.26

.82

:~

.48

.61;3J

.88

.42

.40

Ash, black (Frarfnue nf )----------FAsh, commerofafwbIte FmAnua sfw.

Basaweod (Tfffa glabre .-..–—..iBeA CFSKUSerandlfol a)

Bimb (Betrda MICkry, black (Pmmre aemtfnCottonwood PoPulrmdeftoldew-------------Eln& rook

@muaracamoea)----- .—

C?um,red ( I aldamber st ~rm)d ..-.Hinkmy (tme%okorkaMahogarry, ‘A frfceJI” ?J$s $::_.::::g$~U!~~gwk~@J’ . . . . . . . . . . .

Oak, wmmemial white and m

.....1.)6.,

..

...,..-.—I

) ------------------rp,)7 . . .._._

la~----------

,...-.,)8-. .........

%L!m

1,2404mem

1. . . . . . . . . . . . . .!d U (Q~e~~

9.0 7, Em7.17.1 lf4 m

lJW3

h7 two4.9 4700

fygd lai

IX 6

1?.:

L :3 1,g

1,m , l,(ml%

.82

.29IPo?L),-*G-Efi&YGiTdpum) ... . .

bWahmt, bIaok nglarranfgra)- . . . -----------

soFIwooms (mNI17iM)

Cedar, fnornee (L4bcedroa demrmrreCedar, nortbam white (Thrd8 ocdda!~~~j~Ceder, Port Orford (OhmnmeYParIsIaw@mI-

—------

CYPMSS,muthem (T~imn dtetkinrm). . ..C~~,-==~;-{~-{~h=fiIIcaW) ....-. -..--’

DougbMflr (F%endotsPfnb northern whfte ?Mm5%%!Gj.;-_11Pfn* Norwa (Pfnri5 Mama)--------------Ph. sugar ($fonehlrnbsrtfana)_. . . . . . . . . ..-lPine, wesWrrrwhlta (Pfnne montIwIa).._–.-lSpmce (Picas Wp.)ll . . . . ..--. --.----. ---..-.!

andtuWI 610

l,OW : 7rio680

I*E :1,W6 : ?0

ml1,0S6, H

Slsli m

840: 7%

430m

.40

.81

.48

.46

.84

:ti.26.36

a9 7,m6J &Ml6.1 7,NM7.8 8,~6.0 &ml7.26.6

&m&m

7.4 fi!fcsl7.4 &m

—: ——YThe amrsgevafrwa forflker strw et elastfcllmft andmodulnaofmptare fnetetfo beudi~auddberstrmsat ebwtiollmlt andmx[m,~m-h[ngstrenfi hlnmmp-[on

nnrdld h mra[nhave beanmrrltlpliedby two fectme to obteln vslues for usein ddgn. A atmaerrtof thaw fsetorwand of the remmm for tbek U% fdows: It WM thonghtL?rw for me in flesh. h eflow for the vcrfabll[ty of wood aud the @@ that a grader numbw of values are below the aversge tharrsbers lL

, frmmmcv mmm} wm mmmdfrrdvdmlded rmrmM tha hsda for ddmr llanms. Fmm a etudy of the rathm

‘“ ~The velueativen-sre W percent of the av -~ a~ezant modrdm of efastiol~(l%)ae obta!nad byenhstitutfm-rman with Ionrfat the centerh the formtds E.-P 49 The uaeof tbms valrraso E.b.rthe nerrafformufaswill afvs U

~ with-Speofel Rehmr.ke to She= Defmrnatfons should be ❑W Tbfe formula”lrr-Ah In shear. Vahm of E. mav be obhlrmd by ad nr 10nor cent b tbe valum of E.ae

fittings. Beyond the eh.vfITlfhrft the load contfnrm-ti increaai slowly hntii the deformdfon iilaushbG becomew smera is h seertfae. Pfmrea fn tfde Mhmm wwe obtnlnedbv anrdvim e duration of etrm fsdor of 1.17(seenote I) to the avanwe afmtic limit stres ant then addina 3-...—

alma comparable to thoeafor bendifi” ~-fi~on pemdlef‘b” ai~end shenrw-Mad fn ~~- +0~~- -a% PM cent to ret

‘dPVv~ea frrtbfe columnare fw usefn mmprstlngresfetenceof Mrmre 10 Itudinal shear. TbeY...% %eIwa probebaesueaof thevarfeblfffty in strength end fn order that fsffure b shearmaY

Lble than fsflmw fmm otfm08uie.i2%rthra%ora tds%m shown that MUM

of the favorable Im2uenoen on the dfatribntIon of etresw result g from Hmftfng .shearfngdeformations the maximum et.zaugth-welht re.homd mlntmum vexIabllfty inf

‘%clud~whhurh (F.amerfoena),grem aeb m.~-lvmlmwmhh),mdblue~%. rl~@ate).fem attalmedwheu md box beamsareEOprqmrtioned that the nltfmcte shearlugstre tb tenot developed and faflure by e ear dcas not wmrr.

~Incfudes sweet bfroh (B. Ienta) and ellow bfrok m ~-+-i~Inchdee bfglmf sbwbark hfekory (~ ~cin~j~Inrdudmmatarfel fmm Csntrel America andn Intidw white oak (Q. albe~ h oak (Q. ~

70,..d -k (n I.... w)r.) -+-. . . {n =1-} -

,U= oab~~nedbv multinlylrm averace values bYO.7L Tbls facti fJused

~ ~=ti”~%t hickory (H. anra), frfmruthickory(H.efebra),andshagbarkhkkory(H. Omta).Cuba.

-.-.- ,y. =...-,, .na=%ke-p *=’nut %%%%r (~ phellos),rtnns post oak (Q. etellat4,red cak Q. boradfe),sonthrm red oak (Q. mbra)

‘au., UOa,q. J!.UA,IU-,, (Q. ~Qd=folfS), d,u Inolndw M epmm 7P. rnbra), white epmm (P. glerrca),and ltka spmce (P. sltebm).”

and yellow oak (Q. v rrtlna).

OTHER PROPERTIESNEEDEDIN AIRCRAFTDESIGN

Not all of the data obtained from standard trots oneach specks are of importance in aircraft design, sothat only the pertinent res@ts of the work of theForest Pfoducts L&%oratory have been included inTable I. On thO other hand, data are lacking oncertain properties that might otherwise be expectedin the table. A brief discussion of the omitted proper-ties folIows:

TENSILE STRESS

In general, the tensile strength of wood along thegrain is little needed in the design of wood parts and,consequently, very little information on this property

is available, Furthermore, experience has demon-strated that it is &fEcuIt, if not impmsibkj to getreliable data on tension along the grain. The kststhat have been made show that tho tensile strength,when not affected by other factom, considerablyexceeds the modulus of rupture. Eknco, the valuesfor modulus of rupture may safely be used when ten-sion parallel to grain figures are necessary.

TORS1ONAL PROPERTIES

The torsional strength of wood has been studied butJittle, excepting Sitka spruce. The avaiIable rcsuh,however, indicate that, in designing for shearingstress at maximum torsional load, values one-third

MRCRAFI! WOODS: TEEIB PROPERTIES, SJ31iECTION,AND CHAIL4CCERISTICS 4.73 —●

greaterthan the figures of Table I for shearing strength these factors on the properties and the methods ofparaUeIto grain, which apply to horizontal shear in adjustment employed.

..—

beams, may be used. For example, 1,000 pounds per ,—.-

square inch may be used as the torsional shearing !MOISTURE CONTEXT

.—

stress for spruce instead of the 750 pounds per square The t.able of stress values is based on a M per centinch given in the table under the cohmm heading , moisture-content condition of the wood. This value ._._ .!’Shemin.g strength paraIIelto grain.” For a 3-secand ~ was selected as the redt of a survey by Heim andduration of stress, the fiber stress at ehstic limit in ~ Hankinson of actual service conditions in various parts —-torsion may be taken for any species astwo-thirds of the shearing stress at maxi-mum torsional load.

The mean modulus of rigidity of spruceis eqmd to the modulus of elasticity alongthe grain divided by 15.51or 34,000 poundsper square -inch. The ratios between thesetwo mochdi have not been detitely ob-tained for other species, but scattered testsshow a range of values between 14 and18. Until more dtite information isavailable, the Forest Products Laboratoryrecommends that a ratio slightly higherthan that for spruce be used for otherspecies. A ratio of 17 appears consecra-tive for the purpose.

FORM FACTORS

‘17hedata obtained in test for a givenbeam section are not strictly applicable toanother size or form. When it becomesnecessaryto apply thede&gn vahes of Table1, which are based on standard specimens 2by 2 inches in cross section, to oddl~shapedsections such as the I and box types, a cor-rectionfactoror” form factor” must be used.Thk form factor maybe as Iow as 0.5 forsome extreme sections in I and box beams,whereas it is greater than unity in somecases, asevidenced by the fact that a squareon edge and a circular section carry thesame load in bending as a square of equaIarea tested in the USUSIflatwise position.Studies at the Forest Products Laboratoryhave led to the development of formuIas fordetermining form factors of sedions of va-rious shapes. Detailed information con-cerning form factors may be found inNational Advisory Committee for Aero-

fsooo

I@@Lcqend

A-Modulus of Ruw%re =14880x M‘a014M13000

m

B- sf~es~ af Efask_c Llhi+ in Jfafic Bemfinq =

A/0290 Xlo-“”0i8H

fzooo C -Maxinrm Cru.fhing Sfreqfh Fatw//e/ fo A+eGroin = /02.20x 10 ‘0.0’9M

7000

A

\ \

60G0

5000

moo \

Zwq I f 1 r 1 ,

I I I

Dmoo--

FIcms I.—The rslstion between mechadcd prorxmtfesand the rnotsture content of smaII, cl-

nautics Reports Nos. 180 and 181. (References 5and 6.)

ADJWSTM12NTOF TABLEI VALUES t

The strength figures for design in Table I are notrecorded average vahes from test, but have beenadjusted for various factors to make them applicable ‘to the conditions of aircraft use. The foIIowingdiscussion will bring out more clearly the effect of

-em ofSttkssvmc@

of the country. In this survey the low& averagemoisture content observed was 9.8 per cent for materkdat San Antonio, ‘Ik., and the highest was 15.3 percent for specimens in Seattle, Wash. The generalaverage for all stationa and species was about 12 percent. Individual specimens, of course, showed values

.—

higher than these averages, and stiIIhigher values are ...—probable for extreme conditions. Lcmgdist antefliihts bring aircraft into contact with a wide range

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474 REPORT NATIONAL ADVISORYCOMWTI’EE FOR AERONAUTICS.

of moisture and relative humidity conditions, varyingfrom those of dry interiors to those of the humid coastregions, and year-round service often causes the sameresult. In fixing the moisture content on which tobase the values for de&n, most consideration must begiven the higher moisture conditions that may beencountered; in consequence, 15 per cent was adoptedas standard for general use.

The strength of clear wood in the emalI sizes andcross-sectional shapes common in aircraft is greatly

cavities is the fit ta move out to the surface, wherethq air carries it away. The fiber-saturation point isthat point at which all the free water has left tho cdlcavities of the wood whiIe the cdl waIIs are still satu-rated with moisture. The fiber-saturation point varieswith the species. For most species tha moisturo con-tent at fiber saturation is from !9 to 30 per cent of the.weight of the dry wood.)

Wood is a hydroscopic materirtl,continually givingoff or taking on moisture in accordance with tho prc-

60

055

0;I

I50

0

A=

/0 # . ,,. xI

1 0I5

0I I \I 1 0 0 0I I

- -0-0 0w

470Q 5500 6300 “- 7100 7900 8700 9500MODULUSOf RUPTURE (POUNDSPER SQUARE INCH)

EIQCRE 2.—The vrnlabllfty in the modulusof mptnre of greenSftka spruceasshownby 1,37S@sts. All asroulmennwere cknr, but in selwtlng thornno attontlon WM@d h epemfffogravfty or b pdfon fn the tree

affected by its moisture condition. Figure 1 repre-sents the relation between strength and moisture con-tent of Sitka spruce for four different properties.

When green or unseasoned wood loses moisture,with most species there is no Shrinkage or change instrength properties until the fiber-saturation point isreached. As the drying proceeds below. this point thereduction in moisture causesa stiffening and a strength-ening of the celI waIIs; the compacting of wood sub-stance into a snder volume as a result of shrinkageis another, although a less important, factor in theincreaw’ in strength. (Fig. 1.) (Green wood or woodthat has had prolonged sgaking usuaUy containsabsorbed water within the cell walls and free water inthe cell cavities. In drying, the free water in the cell

vailing relative humidity and temperature conditimsto which it is exposed. Such moisturo changes, ofcourse, may be redueed by applying protective cont-ings to the finished parts, but in timo some moisturochanges can be sxpeckd &fter manufacture, ovenwith coated wood. If dry wood reabsorbs moisture, “ite strength is lowered by about the same amount thatthe strength is jncreased with a similar reduction inmoisture content,

It is evident, therefore, that the moisture content ofwood is an important factor in the strength of aircrnftmembers, and hence in design. By means of relationsderived from F@me 1, Tablo I, and shnilar data, itis possible to estima~ closely the strength at anymoisture condition of the material.

AIRCIW?T WOODS: THEIR PROPERTIES, SEIJN2’ITON,AND CEAIL4CTERISTICS 475 -. —

VARIABILITY

The variability in the strength of cleaz, sound,straight-grained wood may be attributed primarily todMerences in its specific grawity, since in any speeiesthere is a fairIy close correlation between specificgravity and the different mechanical properties. Forwood, frequency curves of mechanical properties areccrnmonIy skewed, more values falling below theaverage than above it. Such skewness results pri-msriIy because most properties increase more rapidIythan the specilic gravity, but is accentuated somewhatby a alight skewness in the spe&c grmit+y curvesthemselves. (Fig. 2 and Table II.)

It was thought best in arriving at design values toreduce the average fiber stress at elastic Emit andthe modulus of rupture in static bending, and theaverage fiber stress at elastic limit and the maximumcrushing strength in compression parsllel to the grainby 6 per cent. The moduhs of elasticity, beingsomewhat more mriable than modulus of ruptureand rnasimum crushing strength, was reduced by 8per cent. The reason for these reductions is to put thevalues for wood on the mme basis of reliability as tho~efor other materiaIs. It so happens that tho 6 percent reduction in moduhs of rupture makes the designvalue correspond closdy to the mode of the frequencycurve, as may be seen in F-e 2. Shearing strengthparallel to grain was reduced 25 per cent because ofvariability in strength and in order that failure byshear may be Iess probable than that. from othercauses. For the other properties of Table I theaverage test values were used, except compressionperpendicular to grain.

A frequency distribution of the spedlc gratit.y(for the weight when the wood is oven dry and thevohne when it is green) of Sitka spruce, togetherwith a bar diagram and a normaI frequency curve areshown in Table H. The crdcrdatedprobable variationin specific gravity of W.ka spruce, assuming a normaIfrequency distribution, is 7.5 per cent.

DURA~ON OF STRESS

The length of time a load is alIowed to remain on awooden member is a very important factor in load-carrying capacity and consequently in tie strwa atwhich failure will occur- It is a well-knovm fact thatloads which are carried safely for a few seconds maycause failure if applied for a long period. It is thisfact that makes it permiasible,in heavy-timber deeign,to neglect impact resulting from moving loads whenthe impact does not exceed the live load producing theimpact.

In the general standard laboratory static bendingtest several minutes elapse after the application of theinitial load and before the maximum load is reached,whereas the masimum stress on a main structural pertof au airpb.ne, such as that occurring in a sharp pullout of a dive or other maneuver, is mant.sined for only

a few seconds. In aircraft construction a 3-semndduration of stresshas be~ aEsumedfor design purposesand the vaha obtained from test have been adjustedfor this cwndition in Table I, for the foIlow@ proper-ties: Fiber stress at elastic limit and modulus of rup-ture in static bending, fiber stress at eIastic limit andmaximum ernshing strength in compression .paraHeIto grain, and fiber stress at elastic Iimit in compressionperpendicuhr to grain. The factor used for this adjust-ment was 1.17.

Curve A of F- 3 shows the relation between fiberstress at elastic limit in static bending and duration ofatrw whiIe curvwB represents the corresponding rela-tion for modulus of rupture. The stress valum asordinatw are espressed as percentages of the value(a, 100 per cent) obtained in the standard static bend-ing test. (Reference 1.) With loads of short dura-tion the members withstand higher stres than in thestatic bending test, and continuously appfied loadsbut slightly greater than those required to stress thematerial to the elastic limit in the standard staticbending test wilI ultimately cause faihre. Thesecurves viere used in determiningg the adjustment factorof 1.17.

SELECTIONOF MATERIAL

The success with which wood has been employed forthe exac@~ requirements of aircraft use bears testi-mony to the precision that may be obtained in itsdesign and manufacture- This succeshd use may becredited directly to an intimate knowledge of thefactms affecting strength, and to the emp~oyment ofproper selection methods. It is the understshding ofthese faotors that makes possible a design precision inwooden members comparing favorably with the preci-sion in other materials.

The chief merits of wood for aircraft constructionare a high ratio of strength to weight; an inherentIightne= in vieight which, for a given depth of member,permits considerable width to tiord lateral stabilityagainst bucldhg; the ease with wtich it can be manu-factured and assembled and, for the same reason, theease with which it may be repaired without eitherhighly skilled labor or special equipment; & relativeoheapness; its ad~ptability to production in sma.Hplants; and the readin~ with which it may be ghmdand spliced. In addition, the present knowledge ofwood permits sekction so as to obtain both the uni-formity necessary for quantity production and theproperties that are best for the work in hand.

On the other hand, the principal factors tending torestrict the use of wood are a not urdimited supply ofthe most desirable species; a hygroscopicity thatresults in” shrinkhg and swelling and changes instrength; and a wide difference in properties with dif-ferent directions of the grain.

The use of the TabIe 1 stresses presupposes thatsdected material will be empIoyed, in order to maintain

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476 REPORT NATIONAL ADVISORY

high structural strength with a minimum of weight.The selection of suitable material involves il.xi.ngaminimum specific gravity requirement, setting a tol-erance on certain defects, and prohibiting others.These may be regarded as primary considerations. Inaddition, there are a number of factors more or lessdirectly related to wood procurement, inspection, anduse that are not ordinarily included in a spetication

COMMITTEEFOR AEROliAtJTICS

point of (1) d.inferencesbetwwm spmios, aml (2] ditTcr-ences between pieces of the Samospecks. ~onskkringdifTerentspecies, the general relation of specific gravityto strength is illustrated by two widely dlflorcnt woods,mastic, a dense Florida species, and balsa, a vcuy light ,central American species, E.ndwke compr&sion testson green material gave the results of Tab10 111, whichshow that mastic had nine times tha average specific

TABLE ~~.—l?esuits of apem”jc gravity detmnina.!iona on ~,106 eample~ oj Sifka spruce

Specific gravityk~z

Pieces in group : VARIABILITY DIAGRAMgroup limits :------ —-—-- . ..-—-. :

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proper, but which will be discussedIa.term contributinginformation relating to the use of wood in aircraft.

PRIMARY FACTORS IN SELECTING WOOD

SPECIFIC GRAVITY-STRENGTH RELATIONS

Studies at the Forest Products Laboratory haveshown that the specific gravity of wood substance isnearly the same for all species,-andhas a value of about1.5, Since the bulk specific gravity of wood is lessthan unity for most species, it is evident that a con-siderable portion of the volume of a piece of wood isoccupied by the various cell catities and pores. Forthese reasons, the specific gravity of “oven-dry wood isan excellent index of the amount of wood substancepresent, and henm of the strength properties.

The relation of speciilc gravity to the mechanicalproperties of wood may be considered from the stand-

,

gravity of balsa, and was also nine times as high incrushing strength along the grain. Weight for w-&ghtthe endwise crushing strengths of them diverse spccicsare substantially equal,

TABLE 111.-A comparison of the apecijii grav-i(iea and thestrength valuea of two uidely differen t tnoocb in the green condition .

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478 REPORT NATIONAL ADVISORY “

Crushing strength parallel to grain and shrinbge areexampIes of properties that vary directly with thespecific gravity. Modulus of rupture, on the otherhand, varies from one species h another as the 1%power of the specitlc gravity. Other properties arerelated to specific gravity by equations of dill higherpowem; for instance, the exponent of specific gratityfor the variation in hardnew is 2)4. It is etident,therefore, that small differences in specific gravitymay result in large dillerences in certain strengthproperties,

Speciilc gravity affords an index of strength aIso forditlerent pieces of the same species, In fact, therelationship is closer than that between the averagesof dif%rent species. Furthermore, the curve repre-senting the relationship of pieces within a species isusually of a slightly higher power than that representingthe average values for dHerent species. (Figs.4 and 5.)

Some species of wood contain relatively hugeamounts of resins, gums, and extractives, which, ofcourse, add to the weight but do not contribute to thestrength as would a like amount of wood substance.Furthermorej the dif7erentspecies of wood vary some-what in the structural arrangement of the fibers. Forthesereasons it is apparent that two species which may

be identicaI in specific gravity may exhibit differentaverage strength characteristics. This fact is illus-tmted by the scattering of points in Figure 4. ““Hencethe specific gravity relationship should be taken as agencmd trend rather than a perfectly uniform law. Adepmtire from tho generrd curve that applies to most

species usually indicates some exceptional character-

istic of a specias, which may make it particularly

desirable for certain use requirements. (The term

extractivee is used for the compounds that can be

removed from the wood of some species by passing cold

or hot water, aIcohol, or other solvent through it

when it is in the form of sarvdust. lZxtractives may

be reforrecl to in terms of the solvent used, such ashobwater extractives, for example.)

MINIMUMSPECIFICGRAVITYIZEQUTREMENT

The minim~ strength values that maybe expected

from random stock of any species may be materially

raised by eliminating a relatively small portion of the

material. Tfi is accomplished by - a bum

speci6c gravity requirement (Table I) as one of the

specifications for aircraft wood, thus rejecting light-

weight stocJL The inspection can usually be made

satisfactorily by visual examination, but in certain

cases it may be desirable or even necessary to resort

to actual specfic gravity determinations, Such de-

terminations made from time to time am of valuo toaircraft inspectors in familiarizing them with the~elation between appearance and specifm gravity.

While no maximum spcc.fic gravity limitations arenecessa~, it is worthy of note that gmatcr uniformityof might and strength can be obtained by removingthe exceptionally dense stock.

PERMISSIBLEDEFECTS

CROSSGRAIN.Cross grain may be regarded as any deviation of the

grain from parallelism with the axis of a piece. It maybe classtied as diagonal or spiral, or a combination ofboth. Diagonal grain is that producod when the sawcut is not parallel ta the bark; spiral grain rostdtefroma spiral arrangement of the fibers in the tree. DiagonaIand spiral grain can be avoided to a largo exhnt bycare in sawing, provided the logs are straight and notbadly spiraled. Diagonal grain is largely avoidcil bysawing parallel to the bark, while spiral grnin may beavoided by edging plain-sawed boards parallol tu [hegrain. Such edging is somowhat difhult, sinco thediraction of the spiral grain can not aIways bo rcatiilydetected. The effect of diagonal and of spiral grain ofa given slope is the same, so that no distinction isnecessary except in regard to cause and method ofdetection.

In order to correlate cross grain with tho strengthproperties of the timber, it is nocossary to Lavo somemethod of measurement. This is furnished by thesinglebetween the direc~ion of the fibers and the edgeor axis of a piece. The angle is usually express-cdas aslope; for instance 1 in 15, or 1 to 15, means that in adistance of 15inches the grain deviates 1 inch from thoedge gf the piece.

A seriesof testsmade at the I?orcst Products Labora-tory on Sitka spruce, Douglas fir, and commercialwhite ash has shown that thc several strcngth propor-tiee differ in the degree to which they aro affected bycross grain and that for properties materially aflectudthe tqdency of values to faHoff occurs with even slightdeviations. (Reference 11.) Table IV presents somoof the remdtsof these ksta, the values representing thoaverage percentage deficiency for various slopes ofcross-grained material when it is free from checks andother defects, in terms of str@h&grtiincd stock,mea 6 and 7 present the results for white tishincurve form.

The weakening effect of cross grain results from tlmtide dfierence in properties of wood along and acrossthe grain. Cross grain is accompanied by m increasedvariability of the properties.

.41RCR4J?I?WOODS: THEZR PROPER~S, SELECTION, AND CHAR.4CTERISTICS 479

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480 REPORT NATIONAL ADVISORYC03@7?17EE FOR AERONAUTICS

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FIctuRZL-The reletion of mdrlne of ru@ure to E@flo m%vlty for emdf, clem spdmem of wh[tu * te8tudfn e.mu @xMf-tion; the epe=ddosrav[ty valufa ore bad on the weight of the wood when oven dry and its volume when gru

TABLE lV.—AUerageperoentqe deP

.ency in atren@h pro~er-tiee with respect tu i+traigti-graine material, of apwal~amcziad diagon-al-grained material of fYariou8 .dopee

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In bending, the weakening effeettbecomes significantat R slope of about 1 in 20 and increaeca rapidly withimmeasein elope. In general, a slopo of grain greaterthan 1 in 20 should not be permitted in a main struc-turedaircraft member. In parts where failure in serv-ice would have no effect on the structure as a whole,a sfope of 1 in 15 is allowable.

DEFEHS CAUSING CROSS GRAIN AND LIMITEDTHEREBY,

@~addition to cross grain itself thero are a number. ..of diifects that am limited in specificatio~” @ncipallYbeiause of the discontinuity of grain and the crossgrt+n that they cause, These defects are knots, pitchpockets, wavy grain, curly grain, interlocked grain, andind@ed rings.

KYKIts.-A knot is a portion of a branch thtit hasbecome ,incorporated in the body of a tree. Soundknots are invmiably denser than the. adjacent wood.

AIRCRA.FJ7 ‘FOODS: TKEIR PROPERMS, SEUEW’I!ION, AND CH4.R4~RLSlZCS 4s1 . —

Knots one-fourth inch in diameter may be permitted length and not to exceed one-eighth inch in width ornear the neutral asis and sIong the middle of the top depth may be aUowed in any part of the section,and bottom surfaces of main structud members, except the outer quarters of the flanges, provided thatprovided they are not closer together than 10 inches they do not cause a slope of grain steeper than 1 in 25and do not cause a di~ergence of grain at the edges of in the ‘outer quarters of the flanges. No pitch pocketsthe member greater than 1 in 20. ‘Where leas aevexe me to be allowed in the outer quart-m of the flanges.strength requirements permit cross grain having a (3) At points where the computed stress, multiplied _slope not greater than 1 in 15, knots up to one-half inch by the load factor, does not exceed 70 per cent of thein &amet& m%y be used providedthey are not closer tugether thtm20 inches, and do not cause diTff-

gence of the grain at the edges of

the member greater tkum 1 in 15.Knots in clusters are often accom-panied by irreguhrgrain and shouldnot be permitted.

Pi#c7Lpoc~ets.—piitchpockets areopenings in the grain of the roodthat contain more or less pitch orbark. Tests at the Forest ProductsLaboratory A.ow that while theeffect of smaII pitch pockets isusuaIIy s~~ht, large ones noticeablyaffect the strength, particularlywhen they are on the compressionfkmge of a beam. AIthough pitchpockets may be permitted on thesame basis as hots, the method ofmeasurement will difter. The lim-itation in both cases takes intocomcideration the cross section ofthe beam occupied and the degreeof irregular grain caused. Thepresence of pitch pockets in Iargemembers is often indicative ofshakes or [ack of bond between therings.

The following suggested generallimitations of pitch pockets in wingbeams of I section (solid or built-up) are based not only on the t~tsjust referred to but also on manyyears’ observation, by members ofthe staff of the Forest ProductsLaboratory, of the effect of pitchpockets and various other defects

— .—

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FLGUM &—The eilwt of spfmland ofd@oMIgrsIn on fibef-at elssilclimi~molulus ofmptureand mndubmOfektidw In eWhbendfngtsstsofsm~ cleu~mems ofvhit.amh

on strength proper&e and on failures under test.(Reference 9.)

(1) At points where the computed stress multipliedby the load factor is equal to the maximum allowablestress, the beams must be entirely free horn pitchpockets. (“Load factor” is qhined in the intro-duction.)

(2) At points where the computed stress multipliedby the load factur doea not exceed 90 per cent of themaximum aIIowable stress, pitch pockets 1Z inches in

mtium allowable stress, pitch pockets 2 inches inlength and not to exceed one-fourth inch in width ordepth may be permitted anywhere in the section,except in the outer quarters of the flanges, pravidedthat they do not cause a slope of grain steeper than Iin 20 in the outer quarters of the flanges. No pitchpockets are to be allowed in the outer quarters of theflq.~.

(4) At points where the computed stress, multipliedby the load factor, does not exceed 50 per cent of the

482 REPORT NATIONAL ADVISORY COMMI’rrEE FOR AERONAUTICS

masimum allowable stress, pitch pockets 1X inches inlength and one-fourth inch in width or depth may occurin the outer quarters of the flanges and pitch pockets3 inches in length and one-fourth. inch in width ordepth may occur in any other portion of the section,provided that they do not ca~e a slope of grain steeperthan 1 in 15 in the outer quarters of the.flanges.

Wavy, curly, and interlocked grain.—Wavy, curly,and interlocked grfiin are snbject to the same linlita-tions as other forms of cross grain.

Indented growth ring~.-Indcntcd rings of annualgrowth appear as slight local depressions of the ringscmend sections of pieces of some species. On a tangen-tial surface they app~ar as longitudinal scars, which

gives rim to the t.ernl“ bear scratches”that is sometimes fipplicd to them.Their eflcct is Ices than that of thopermissible knot and may be ignoredin members of appreciable size,

SHAKES AND CIIECKS.

Both shakes and checks m separa-tions along tho grain of the wood.With a shako the grentcr pnrt uf thoseparation occurs lmtmen the ringsof annualgrowth, whereaswith a checkthegrcatarpart of thoscpmution occursacross the rings of annual growth.Checks and shakes me more commonin large timbers t.hm in smalI piccxs.The weakening oficct in rmy cmo isusually greater than thtit titlributfddeto the visiblo extrmt of tho opining.Aircraft material containing shakosshould bo rejected. Aircraft lumhcrshould be relatively frco from rhccks,and finished aircr~ft parts containingchecks should be rcjectcd.

MINERAL STREAKS.

Ifineral streaks arc localized accu-mulations of mineral matter somo-timcs found in wood tindam generally=ocifitcd with a slighLinjury to tholiving trco, such 8s a bird peck throughthe growth layers immc.diatcly underthe bark. Mineral streaks in thmn-selvm appear not to affect strength,but me sometimes accompanied bydecay and consequently pioc~scontain-

*SLOPE OF GI?.4114 ing such streaks should bo impccbxi

FIGURE 7.–The eflmt of wknl and of dhsond @n m the maxfmum h@t of dmP ~ ~@ b~dfu ~ for this clefect.. ~ matcrial COIIl.aiII-ofsmell,clear8pm!mmeofwhite@

(6) Pitch pockets in the web may not be closertogether than 20 inches; if in the same annual growthring they may not be closer together than 40 inches.In other portions of the section these distances maybe10 and 20 inches, respectively.

In measuring pitch pockets in a piece the lengthis taken as the surface dimension in the direction of thegrain, the width as the surface dimension across thegrain, and the depth as the maximum distance thepitch pocket extends into the piece.

ing decay is rejected. Sornciimosmineral streaks are present in sufhient quantitiesto be a factor in the dulling”of woodworking tools.

INJUiY RESULTING FROM INSECT ATTACK.

Ir&cts sometimes cause holes in lumber. In esti-mating the collecton strength thoso holes can IJOcon-sidered from the standpoint of the amount of materialremoyod. men the holes have resulted from insects.that attack the green or partially soasonod wood,their presence is usually detected easily. On tho

-CFLHT WOODS: ~IR PROPERTIES,EYEIiECTION,MTD OHKRACT!EBISTIOS 483

other hand, the presence of pow&r-post beetles, mostof which contlne their attack to dry lnmber, usuaUytie sapvmod,, can be detected in stored stock onlythrough the most cmeful examination; at times cut-ting into suspected stock is necessary. Hickory, ash,oak, and black walnut lumber are most subject h suchattack, but other species are not immune. Lom.scaused by Lyctus beet.b, which are perhaps the mostcommon powder-post beetles, can be prevented throughproper methods of CIassiflcationand the piling of stockby kinds; by separating heartwood and sapwoodstock, when possible; by periodical inspection; byutihzing the older stack &t; and by coating thelumber to C1OSSthe por&, thus preventing egg depo-sition. W&m material is under attack, proper heattreatment in a IciIn at a temperature of 180° F. willkdl the borers; two to three hours should be suilicient.(References 4 and 7.) There appears to be littIedanger of powder-post beetle attack in assembIedaircraft parts, however, since the finishes applied areefTectivein preventing entrance.

SEASONING DEFECTS.

Kiln drying, if properly done, can be accompKshedwithout injm y to the strength properties of the wood,tests at the Forest Products Laboratory show. &tefer-ence 10.) Furthermorej results can be obtained thatfulIy equal those of air drying under the best condi-tions. On the other hand, fa~orable appearance ofstock alone is not a suitable criterion as to whetherthe stock has been injured in the drying process. Thebest method of obtaining satisfactorily dried stockis to use an approved drying schedule, such asone of those recommended by the Forest ProductiLaboratory.

The seasoning of wood in its simplest terms ccmsistsof driving off the excess moisture always prwent in theliving tree. To acoomplkh this, a moisture differencemust be established between the interior of a piece andthe outer surface from which the atmosphere takeaaway the moisture. If the drying is too rapid, toogreat a moisture difference wiII exist, and the redtingunequaI shrinkage @ cause stresses that manifestthemselves in checking, honeycombing, casehardening,or warping. Collapse is another defect that sometimesoccurs in seasoning, e9peciaIIy with certain species,in spite of care.

Wood that is checked, honeycombed, or collapsed isnot suitable for aircraft construction. That whichis casehardened or warped maybe accepted or rejectedas det=mined by the severity of the defeot and theuse of the stock.

Storage after kiln dr@ng.-Good practice in woodfabrication invohs the storage of Iumber under cwrerfor a short period aftar it Ieaves the dry IciIns andbefore complete manufacture, to permit the reLiefofinternal stresses that may exist, and the further

41680-31+

equalization of moisture content. Such stock shouldbe bulk piled, and heated storage space is desirableand may become essential as methods are refined tomeet more meting specifications.

h~A.NUFACWWE.INGDEFEOZS.

Manufacturing defects are only rarely the cause ofrejection of aircraft parts, since strength and notappearance is the criterion of acceptance. The defectsthat 8re most IikeIy to result from manufacture are aalight raising of the grain and splintering in the machin-ing process. The spktering that sometinxs occursin routing a beam when the shaper Imife finishes outagainst the grain should not cause rejection unIess thesplintering extends into the fkqys. Excessive sandingta produce a smootldy iinished surface shouId beavoided. The finish of a wooden aircraft memberneed ordy be suf%cient h produce a good surface forvarnishing.

PROHIBITED DEFECTSDECAY.

All woods, to a greater or 1sssextent, are subject tostains and decays. The stains not associated withdecay, as a rule, have little tied upon the strengthof wood, whereas decays affect the strength even intheir eaxly stages, and before any appreciable reductionin specific gravity occurs.

The development of fungi is dependent upon a

relatively high moisture content of the wood, suitabletemperature, an abundant supply of food substances,and a supply of o~gen, which may be obtained fromthe air. Lumber that is kept either dry or entirelyimmersed in water wiUnot decay. The most practicalmethod of preventing decay in aircraft lumber is toavoid undue hokiing of logs under conditions favorableto decay, to kiln-dry the hunber ~ediatdy aftersawing, or to &dry it immediately with properprecautions, and then b keep it dry in proper storage.Under such conditions wood can be heId indefitdywithout Ioss of strength or other deterioration. Boreror termite attack, of came, requires its oti pre-ventive measures.

Both stains and decays are usualIy indicated bydiscolorations of the wood, and it is frequently difli-cuIt to distinguish betvieen them during their earlystages. Furthermore, the conditions that are favor-able for the growth of stains, as a rule, are sIso favor-able to the development of wood-d=troying fungi.For these reasons any discolorations in aircraft woodshould be regarded with suspicion and pieces so dis-cdored should be csrefully examined.

In exanillin g discoloration, it is necessary to avoidconfusion with some of the natural color variations ofthe species. For instance, in Sitka spruce tha hea.rt-vrood has usuaIIy a light reddish tinge, dightly dis-tinguishing it from the sapwood. Sonic trees of SMcsspruce, however, have a monounced reddish or brown-

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484 REPORT NATIONW JKDVISOBYcoMMrrrEE FOR AERONAUTICS

ish pink heartwood, which is quite uniform in colorthroughout. The color cMference is striking in a snr-faced board containing both heartwood of this colorand the characteristic white sapwood. This reddishor brownish pink hsartwood is not discolored by decayand so can be safely used for aircraft construction.

Decay varies in its effect on different strength prop-erties. The woo~destroying fungi use certain con-stituents of the ceLIwa~lsof the wood for food, with theresult that these wails are broken down. In the finalstages of development of the decay little or no strengthremains in the wood. In the early stag= the crushingstrength parallel to the grain is affected but little,whereas the shock resistance is reduced markedly.When acceptance is based on specific gravity andappearance alone, ti boards containing any evidenceof decay should be rejected.

COMPRESSIONWOOD. “-Compression wood is a type of abnormal growth

frequently occurring in the under side of leaning treesand limbs of softwood species. When viewed in -thecross section of a log, it may comprise all of the mate-rial on one side of the pith, or may consist merely ofone or more growth layers, lying between growthlayers of normal wood and extending over only a por-tion of the circumference. It is characterized by highspecific gravity, and has the appearance of an excessivegrowth of summer wood. Compression wood in mostspecies shows but little contrast in color between springwood and summer wood. Unlike normal wood, it hasan appreciable endwiee Shrinkage, This causes crook,bow, and twist, particuhrly when compression woodoccurs in combination with normal wood.

Strength tests made at the Forest Products Labora-tory show tha~ compression wood, although usuallyvery dense, is not so high in strength properties for itsweight as normal wood. ThiE fact applies especiallyta the bending and the shock-resisting properties.Il%ile pieces containing compression wood are almostcertain to be rejected because of their tendency tosome form of warp, all such pieces that may come upfor find inspection should be rejected.

COMPRESSIONFNLURES.The defect called &compr~sion failure, as the name

implies, is an injury to the wood resulting from itshaving been overstressedin compression. Such injuriesmay remdt from excessive wind against the standingtree, from felling trees on very rough or very irregularground, or from rough handIing of the loge or lumber.Compression faihnws are characterized by a buoldingof the fibem that appeam as streaks on the surface of apiece substantially at right angles to the grain, Thestreaks vary in degree from those so pronounced as tobe unmistakable to very fine hair I&a that requireclose observation to detect.

Compression, failures aflect the strength propertiesof wood, the degree depending on their magnitudomd l~ation. They have the greatest effect in reduc-ing the shock-resistingproperties. All wood containingcompression failures should be rejected for aircraft uscin parts where strength is important.

SECONDARY FACTO- IN SELECTING WOOII

LOCALITY OF GROWTH

The origin of wood with respect to locality of growthis not ordinarily a suitable critarion for either accept-ing or rejecting stock. Oft.eg the supposed difIcrenccsin the properties of a single species growing in dXcrmtsections of the country are practically insignificant;in fact, there may be more difference in tho strengthsof timber grown in adjacent townships than betweenthe averages for two widely separated regions. Whenconsidering such dMerences it is well to keep in mindthe general principle that wood of a given specificgravity grown in one locahty will usually have thesame strength as material of the same spocics and liliospe@c gravity grown in another locality.

There are a few cases in which tho matmiai froma giyen region will average considerably low-m instrength than that from another, but even in thcsocases the d.iflercnces are in the main reflcckd in cor-responding differences in specific gravity. ThugDouglas iir from the Rocky Mountain region avmagcslower. in weight and strength than that from the“Paci6c Northwest. Likewise, the material from thoswe~ed butts of southern swamp-grown ash, tupdogum, and a few other speciesis much Iighterand weakerthan-lhat from a higher position in tho same trees,and that from northern upland-grcmm trots. Iflumber from southern swamp-grown ash is to be usedin highly stressed aircraft parts, it is csscntial thatthe lumber manufacturer cut off the swollcd portionshorn”“tie logs before they are sawed into lumber.

The facto= affecting tree growth, which are verycomplicated, cause tide variations in the de&ityand the strength of different trees grmm Withinlimited areas and even of the wood put on by a treeduring difTerent periods of its life. Tho effect ofthese immediate environmental factors is vcqy greatand ynithin the natural range of the species it o~er-shadows such factors as geographical location.

‘l@s made at the Forest Products Laboratoryshow~.for instance, that Sitka spruce from Alaska isfully the equal in weight and strength ta that from thoStates of Washington and Oregon at tho scuthwnpart of its range. This and many other observationsindicate that, in the absence of specific data concerningwood from a given source, the general average of allavailable testi on the species is a more reliablo cst.i-mate of the strength properties of the wood from the

ALRCR.AIT WOODS: THEIR PEOPEBTIES, SJ31JIOITOJS,A.?? CEARMWERISTICS 485 .—

source in question than data on sarnpIesfrom a nearbysource or from a site that appears ta be similar.

Sometimes, howeww, facto~ other than densityand strength may cause a preference for mat-erhdhorn one region as against that from tiother. Foursuch factors are lo-g practice, seasoning practice,manufacturing facdities, and the grading methodsempIoyed. These factors, of course, are subject tocontinual change and an immediate contact andknowiedge of conditions is required to permit anyexercise of regional preference on their account.

RATEOFGROWTH

It is impossible b set up a satisfactory rule forappraising strength in terms of rate of growth. Thespecific gravity or the density is a much better criterion.

In coniferous woods, such as spruce, material ofvery rapid growth is most Iikely to be of low densityand hence to fall below the a-mrage in strength. Forthis reason the prase.ntspetications for airoraft sprucerequire that acceptable material shfl ha-re at least sixannual growth rings per inch. Sitka spruce of rela-tively slow growth shows no noticeable increase ordeorease in strength with rate of growth, and hence itseems undesirable to set a Emit for the maximumnumber of rings per inch. W3th conifers in general,however, the wood of exceedingly SIOWgrowth, asfound in the outer part of overmature trees, is verylikely to be of Iower density and strength thaa thatof medium growth. Yet this slow-groti materialworks easily and stays in place well; in manufacture,all slow-growth material is superior ta equivalentstock of rapid growth.

With hardwoods, on the other hand, wood of rapidgrowth is usually above the average in density andstrength. lteverthekss the rate-of-growth standardcan not be applied indiscriminately even to hardwoodsbecause some very rapid-growth material is brash andinferior in weight and strength.

POSITIONIN TREE

As a genend rode,wood of the highest specific gravityis the strongest regardless of its position in the tree.There are exceptions to this rule, but they are confinedmostIy to the lower 10 or 12 feet of the tree. Someslight variations with distance from the pith of the treeha~e been observed that could not be entirely ac-counted for by the difkence in specific gravity, butthey are not of sufhient magnitude to be considered.

HIZAHT~OODMJ13SAPWOOI)

From the standpoint of strength there is, in generaI,no dMerence between heartvvood and sapwood.Although the change from sapwood to heartwood ina few species such as redwood is accompanied by antitration of extractires that increass the specificgravity ad, to some extent, cause an increase incertain strength properties, this factor does not appear

I

to be of any importance in aircraft. Specific gravityor density is the best criterion of stre@h, whetherit be for heartwood or sapwood.

On the other hand, there are important differencesbetween heartwood and sapwood from the standpointsof ease of seasoning, resistxmceto decay, ease of pen-etration with preservatives, and the like. As a whoIe,the sapwood of all species is relati~ely nondurable,whereas the heartwood of certain woods such as PortOrford cedar is higldy decay resistant; the sapwoodof most species, however, permits easier penetrationwith preservatives and o~er agents than heartwood.

TOIJGHXE.SS-TEST METHOD OF SELECTI?JG WOOD

As recounted in this report, wood lhat will meet theexacting demands of airc~t semice can be satisfac-torily s&cted by carefd visual inspection, supple-mented at times by specih gravity determinations.Because of the extensive information on the effect ofcertain factors on strength, no further requirem~tsare esentifd in ordinaqy service.

On the other hand, it is conceivable that there maybe special oases where specific information on thestrength characteristics of the wood of certain mem-bers is desired. For struts and simibir members inthe Euler cohmm CISSS,the load is dependent on thestifhms of the materhd and consequently it is possildeto predetermine strength by means of a simple deflec-tion test, within the ehistic fit, without in any wayinjuring the member. With members in which bend-ing strength enters, however, there is no direct methodthat can be used. Further, to determine the modulusof rupture imrolves the compIete destruction of themember and any attempt to estimate it by pt@iaIloading is likely to produce stresses beyond the elasticlimit that WU uhimately impair the strength of themember and the safety of the structure.

A definite means of appraising strength in such casesis provided by the Forest Products Laboratory tough-ness machine which, through teats on relatively smaIIsamples from a piece, offers a comparatively simplemethod of sekction.

The For=t Products Labratory toughness machine(&. 8) operatea on the pendulum principIe, but itdi&ra essentially from other typ= in that the load isapplied ta the specimen by means of a cable fastenedaround a drum mounted on the axis of the pendulum.h the test a specimen % by % inch or % by % inch incross section, supported over an 8 or 10 inch span, isloaded at the center by means of a tup and a stirrup”that sIips over the specimen.

In brief, the toughnew-test inspection method thatmay be employed for stock intended for exacting usesprovides for the testing of small specimens (usualIyfour) from the phmk from which the part in questionis taken. To be acceptable, the piece (1) must eithermeet a minimum toughness requirement established

.—

--- .

—

—.-x

.“

—.. ——

+— --—

-—

. ..-,“-—

.-

——

.—

486 REPORT NA~ONU AD~ORY

.for the specia under ~mid~,W’nI

or if” within 8

certh t~ler~ce Now ‘e YUm ?’?t .PES haddition the pr=mt spec-fio grawty lu-dation;(2)must show a limitd range in tiwhms valuw for all .-

. The procdme is aimplifid further by the fact~’~~~ the moiat~e condition of the spwimen mQYbe

Iignord, for tests have shown that toughn~s is affectdcffectbut little by moktm and consequently any

specime~ from the same piem; ~d (3) must PSSS 1oarefd visual tipeotion.

In practi~, the prwedure is 1* compEcaW timit appe~ from the description.

Tbe tests are made

wry~rapidly tmd no cslcdation is nec=arY to de_-*e the resu.1~. It is rwcesssW merely to read ths

final angle on the mactie at failure of the spetimand for this value to take the com*Pon@ tou@@(work in inch-po~de per spetimen) direotiy from a

..

FWJBSE,-The For83tprodu@latw~m &oh@ forta+tiWth’atoe 01wood

may be negl~tid.The one essmtid in the apphcation of the toughn~

method, in ad&tion to the net-w mactie for mak-~ me ~ts, is a howledge of the spcciw with respmt~ -urn ~WhQSss requkemati. TCStSmade ,atthe For=t I?roducts Laboratiw on a number of spoc@have served as a b~ for estab~ti such vduw. AS-av of these data are presen~d ‘m Table V,

AIBCBAFT WOODS: -Ill PBOPEBTIES, SELWXTON, AND 0HABACKt73BISTTCS 487

TABLE V.—kfinimum acceptance requirements for aircra tiwoode based tinMS 1in theForeeJProducts Laboratq toug -

long use, may be considered more or less standard.FoIIo& is a list of these woods, together with tbirprincipal uses:

WHITEASH

nem machine

. . ..-

. .., -,-1.,

—.I.Ongerons, propellers, landing geu struts, float-

ribs, reinforcing for structud members, berit workon wings and fuseIages, ch@wj tail skids, cabane struts,bearing bIocke, wing leading edges, float bulkheads,false keeIs, control handks, and fusdage struts.

i

13pecieeofWocd

—

BALSA ——.—PIywood core stock, espeoidy where insulation is

desired, as in cabins, filling, stremdining, ~d ffigstrips.

BASSWOOD

Wii ribs, veneer for plywood, templates, and webs.

Whimnsh-----------------Yellow bir&__—._——

~!&’’;::zz::I:. . . . . . . . ------

BlackWdlllt-.....-.--..

—

—~Thekdtstobeap edtotbetmge-nthlfaeecitkemzdmen.r’~Bu@mwdghtan TohmOof OTendrYwmd.

&=&d:73&’32.wdbk’&=*5&:KP=tw31%!4Mamollg4 Spedmr&

SUITABILITY OF SPECIES

Many speti of wood are sufbiently satisfactoryfor aircraft sertice, even though relatively few =e nowused. Several factors lead te the present concentra-tion, and tend h delay the employment of other species.Among the foremost are the excellent combination ofdesirable properties in the most used species; themanufacturer’s knowkdge of methods of selection andinspection; practical considerations of avaiIabiIity,uniformity of quality, cost, and ease of manufacture;facility in gluing; end ease of drying.

The wood requirements in aircraft axe (1) for material in the form of lumber, and (2) for plywood. Thepossibility of combii speciw of widely diilerentproperties for core stock and face pIiss makes plywoodvery adaptable to a huge range of requirements, andhence its uses in aircraft are so many and so varied thata partial Iist of them is desirable. Some of the morecommon applications, as enumeratd by John F.Hardecker, are as follows: Fuselagej leading edges,engine bearers, flooring, tti linings, center ribs, tankcover, center cover, float parts, box beams, wingbeams, walkway ribs, seats, rudder, drag ribs, end andtail ribs, walkway, head pads, propeller apimmr, wingcovering, step boards, bulkheads or partitions, floatcovering, aiIeron or elevatcr surfaces, instrumentboards, afterdeck bulkheads, webs end wing spars,bracing and gusset plates on fusekges. (Reference 2.)

WOODSNOWCOS~ONINAIECRAFPSEBVICE

Of all the requirements of wood in aircraft, theprocurement of suitable, clear, straigh&grained lumberpresents the most important problem, and the suit-ability of species will be discussed prhnariIy from thisstandpoint. Further, the question of suitability canbest be approached by means of a comparison of otherspecies with the woods now employed, which, through

YELLOWBIBCH .-

Propelkrs, and veneer for p~ywood. .—

MAHOGANY

Deck, bottom, and bulkhead pbmking of hulls andfloats; veneer for plywood, such es that used forwing covering, wing tips, and wing ribs; propelb;control wheels and handles; pattern work; interiorfinish; and instrument boards.

—-SUG= MAPLE

PropeUers, veneer for plywood, jigs and modeIs,assembIy forms, shearing blocks, and bearing bIocks.

OAKPropeIIers.

WHITFIPINE—-.

‘Webs, cap strips, corner blocks, fairing’ drips,patterns, and forms.

YELLOW POPLAB

Veneerfor p@oocL

RED, SITIU, ANDWHITE SPRUCE

Main stictural parts, such es wing beams, struts,and longerons; float and hull constrwtion; ribs, webs,landing gears, cap strips, stiflenem, flooring, planking,and veneer for plywood.

.,---

BLACKWALNU1’

Propellers, cabin fur-, and instrtunent panels.

REQUIREMENTS IN MANUFACTURE —

The bahnce and alignment requisemente for prope.l-Iers are probabIy the most exacting requirements inthe mwmfacture of aircraft. Balance and freedom .from warping in service depend as much upon matchingelI Laminations for density, moisture content, and

.-

--

488 REPORT NATIONAL ADVISORY CO*E FOR AERONAUTICS

direction of annual growth rings as upon the speciesof wood used. Some species give very satisfactoryresults with rapid manufacture and oily a moderatedegree of refiement, while others, such as oak, mustbe careffly handled, with ample time for conditioning,to give the best service.

The preceding statement are Iargely typical incharacter, though possibly not in degree, of all woodaircraft parts, and therefore the specific manufactur-ing requirements of other parts have been given noadditional discussion.

MECHANICAL PROPERTIES OF NITfVE SPE4XES

Table VI presents information on the propertiesof some native species compared with spruce for usein aircraft. (Reference 3.) The comparative valuesof column 7 are suggested as an index of the suitabilityof a specie-swith respect to its mechanical properties,in terms of spruce, in so far as such a relationship maybe expressed by means a of single vake. It isimportant in using the table to consider also the otherindividual properties listed, since a species, to beentirely satisfactory as a spruce substitute, shouldnot-fall decidedly below spruce in any property.

TABLE VL—Properlieaofuatiownaiirewoodswmparedwiththoseof upruce,for aircraftservice1

111sI 1.

spruce,red, S1tka,andwhh L..---------! 1~ ~

HARDWOODS (EILOAD-LEAVED SPECIES)

------+

‘“” ~ P“-)-Appld Ma ucpumlla var~j~::~-.-~~~::::::-Aaa~a)B-~r’e Whtt4 (Fras4mr9b41tmore

. - .--—. —~~ black Frarlnua n@xa). . . . . . . . . . . . . .

LAL@ blue ( aslnua~uadrangoleta)- . . . .

IAe&13r@r (Frerbma perrnaylvantcak4n-

. . . . . . . . . . . . . ---. —--- ---AsL Oregon(Fmxfrmuoregona). . . . . . . ..-A8h,pumpkhr(Frnx4nncprofunda)- . . .Aeh,Whib3(fiu8 SZUtiCUW4). . . .._.. -Aah mmmercialwhita(wmmeo044 SPC-

da . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

100 110:114106 112 m

m 149 946124la g Ig

143 Ma 265lub m 924130 ua 24s149 166 !235

146 m im

ASpen PoPuksskemnIo!des).._....... 96

I ‘

&%;$i?#&J%J:~::g: ,% ,:ijBfrch,Almkawhit-e(’Betulaneoalwkana).. f32 lza 145

Btmh,UMYCSetda PO~Oh)----------- ~

i%% %% ~:::z~::?r!::::::::::: lac $igBfrch, Ye!kw (BetukI Inks). . . . . . . . . . . . . . 149Blnckwood fAvk!mmfa nitfda) . . ..--- . . . . . . .224 lW 1440

144W101

145lae112in

161

6762

2144

@1641s4lslI&a

—10370

699394

86m7694

9194

J!8495

ltial99

%1In tbte tableeaehspruceVSIUCferatal lM parcent and the mraepondfng valueu

d other wood-qare then amramedacmrdlmgly.] These vahreaare averageuobtrdml from the foIIowtngwelghtfr@Column2

VdlSC41.0wirunn4VUIUM1.5 mrdmlnmn 6 vdl~ LO.1Coiumn 6 Yrdmadfvfdad’by the 6/2powerofthe corrmpondingapeclfloE18vftka

(mlnmn 1).4Thabrutovaluesucedfornpruoe4ncohr.unaI ti & Incltiv’a, am 0.S7,72,@ 71,

and ML respectively. TheseVSIUWarctakenor calculatedfromComparativeStrength Pm ort[esof WoodsGrownin the United State%wb4chdfacuwc thefrderivation. &feran. 8.)

TABLE VI.—Propertie~ of uanouanatirc woods comzsared tiththose of apt%ce, for- airmqil service--Continued

HARDWOODS (BROAD-LEAVED SPECIES) -Contfnned

1-

I

1Chemy,b~ack(Wrnui cezotina)...___ IZOkwry, 4n(PnmUs nnaylvlUlka)_...

!8Chastnu (Crutanca ntata) . . . . . . . . . . . . IfiChfn uapIn, golden (CaetanopafsckryaJ-

%aphy }._ .. . . . . . . . . . . . . . . . . . . . . . . . . 114cOttdiWWd,wrn ~O@kSSddmid~) . . 1C4

“/Wckory, sbasbark (HIoortaOusts) _____ 174mck ,wa~(Elcorfacqnat4ca)._.-_..IMHlck%?mI$f&iiyJ$%%%%%2::: %

. . .. . ... .... . . .. ... . . 1s5HoP40r&aam(C4Strykvtrytnkurn). . . . . . . . . S~hkwood (ExOtheep%nfcnlata)-- . . . . . . 107

?i!i%ii!i%%i%!!i%%Fm~l %.. . . . .

1Mane’honey(C4kiffsfiMacanthj..--l.

&Mad a (Arbutus manzfasff).- . . . . . . . . . .bfagnol@ Cuoumber (MagnoIta acu4’Kd-

+&-_Ri&ii--G&ii;iGiRiiC. ..................... ...........

Mrgnolf&monrrtafn(Mcgnolfafraee)._.

la167

119

L24lm

‘“~~p2.Hs’’&=l--- ~~*=eAQrfim%%h)...--...-141

-.. —....

oak, awwmpcheetnut(Qumoc m4nuc)...- 162Oaki$~~ red (Quarcrrsrubm pagodm

- . —.. —--- . . . . . . .. —------- Ku

[

a

74

l!!206Ld!2

[71

1%

14366

f%mla157

44a17!

rti114.4a:Mni7bKa

.. .

..-

.—-.-.,.-

,.-S6,.=C5g

g

mn

36

f

m

{

53

;93624357$6u14fd71uuw46

mw0.446

‘idm4067a46

—

I—6

—

al

1!!6sa

110a642

92w

m

1884w

%

#mwm4979m

Ml126fse

i%fsb

2lau76

Ill-1la-t18J

164lfiw

L!a

m104

lW

1!311678

&1s5

1%181‘ 76Io7’7bIsa ; 117Ml : IsaIm:l!mX44’lta174 n214 lB163 105ml 111

110446117Mb

mg

170Malm

67

~

m

i!!112mw

—.7

—

SEIW!TION, AND CHAFL4CTERISTICS 489AIRCR.&FTWOODS: ~IE PROPERTIES,

T.4BLE YI.—Prop&”es of carious natice rcoods compared with This table is not intended as a Enal decision inregard to the suitability of the species listed. Prob-ably no two engineers could aggee exactIy in such anappraisal. In fact the relative importance of the&tTerent properties varies in the difTerent members,is dependent on design, and changes with the type ofmachine and the speed. Hence, even grantig uni-verwd agreement on the detail of the relative import-ance of various properties, it would stll be dii3icultto compare the difterent species from the standpointof aircraft use as a whole.

The four properties combined to arrive at thesuitability index (column 7] are specific gravity,bending and compressive strength, shock resistance,and stiffness. Shock resistance does not come intoaction untiI the elastic Emit has been fully passed,but is included because of the margin of safety itoffers under extreme conditions. For this reason,of two species equal in other respects, the one ofhigher shock resistance is preferable. In species ofhigh shock resistance, the initkd small compressionfailures in a beam that is stressed to the UItimateusually tend to distribute, and thus more work isabsorbed than can be absorbed when such failuresconcentrate into a single localized failure. A speciesof high shock resistance wll also stand more frequentrepetition and revered of stress of high magnitudethan one Iower in this property.

The weighted and a~ersged figures thus obtainedwere divided by the speoific gravity raised to the 3/2power. In this analysis the consideration of such fac-tors as effect of size on the strength, s~ness, and buck-ling of thin pints, together with the essential require-ment in aircraft of lieep~~ we~ht to a minimum,necessitated the use of a pouer of specific gratityhigher than the fit. Here again judgment wassxercised in selecting the power, as welI es in weightingthe properties.

DETAILED DISCUSSION OF SPECIES

An airplane can be made from practically any

species of wood that d furnish material in the

required sizes, and the size of the pieces required may

be greatiy reduced by laminating and sphing. Byihoo+ng the most suitable species, however, it ispossible to reduce the weight of the plane appreciablyand to increase its efficiency. The following com-ments, therefore, are based not on the bare possibilityof use, but rather on the relative suitability ands.tlkiency. Further, the differences in strength betweegmany of the species are relatively smalI, and lower-ing the quaIity required in any given species is thuslikely to make materiaI of a I@her quality from aninferior species preferable to the low quality fromthe superior species.

Some of the species included in Table VI obviouslyneed not be m“nsideredfor aircraft use, because of the

those of spt%e, fof aircrqft wm”ctiontinuedHARDWOODS (BROADLEAVED SPEClE9J-Ccdnndl

I—

—

mG-lG-

Tl-176110 1456#.40&laooaa

m U9:ln1s9 lw, m5

Oeik$ eommereid TM& (mase of 6 [

=z-&-G-$8bdtimtiti, . . ..-_.-.--_-—-

Palv&MrW (ShnaroukmgIe,nt’&__..--— ——

;=:nmmhimTm VirgInImij-i

tla lw Wo10J 63 a

60 76;% 149 w

166 236

-—

——

%’%%w&?$t%’=:_%!2%x’EEkR%i$i%krzRhododendron.meat(Rhododendronmaw ,

Zw; E43 460g: : I%

10s1 UT wME!El 24s

11-$3143

#L66812114 lol m1651z6199m. mIz7ilmus

I

rm, Io4152lx! m m19s X56 !zIO14S’M4

92[MS3lw16+a,%

M& ls# lEJ

61 m 05Sa w 91 .—

hnum)~9a5afrae@safrasVariifowrnl)=ceb (AmeIMchfereamzzJ

FsuWd3ell OstB~..-_ -------Sourwocdo dendmmftrborelmn)...-.-,

3Stoppef,red ugerdamnfnen)... __ —=

—Suge,rberry(CeltfsIs3vfgat.q_.._.__J

S#&%!J$%?i%d%kj:::-;Wbblnblkue (k%wi%kj:-.------;~flOW, bIaek(SeUx

YWilJJJw,weete.rnbIwk 6&—i—&&:jwItcb-b8r01(HaMaMev!rgIdmlu)__.r

Ea’cil.w146’ m Ils!463! 9s 179,

.-—-—

.,SOFTWOODS(CONE?EES)

+

Cedar, Alaska (Cbmnmyptuis nmtkn-wnsis)-.__.—.__.

Cedar,easternreded~ vir@f@-:.ceder,ineansaCader,ncdhern white hn.&%%lte)

FCe&#I)~ Orford ( hatmeeyparialaw-. .--—

cdaxby~ WbIta(Chmeqqda

Cdar, w6&n ‘tiiThnfa PIkata)-----@P=% ~uthem (Tuodlum di9tiChl@iD- dr (wudotsoga titia) (Caast

DW~ &@iii@i b3dfOh) ~sndEmpire ~)..._.------.-.-._-_-

mEnm

7s

zm117

Iw75

1%75

mIcn

g

%Kg

lit%

#

1%97

ls7lmm

EEi2

i%U7m

K&

E66

2It&

-—

S4

i%

—

—-—

-

E?2

Ill

1Dcmm&(Peeudoksngats%foiis) (EO&F

m#lgil&(~~;fp&a~;–~+_%Fir: Crr.UformArwl(AbM mr@d6&)..__- &Fir, ewkbsrk(Abks arizmdca). . . . .._.._ 76Ftr, lowlandwhita (AblfagrwxIis)____ Xm

i%%%$!xi%!%-:::c::= !!!F& true (aWage of4 @em). -...___. S5

.———

-

~

H=&%%&T?%&%%%&i)-JmIl

, =stom3(T.9uge.beterohytkw.-., 614zscor(Jude rta+blma]-

l..arc~westem (LarIxccddentalls -----

m116106EMm

<

W I=k (Ptms bf-dml...i—————wi#ywJi%%k.g.g$!_::::---gPine, IOdgv@e (PknuYcontmta)..__- 10S

-------

Pine,loogkaf(Plntr9paIuekia)._. . .... . 140

<PbIe,monntaki(PInn3prmgen9)-——.._ WPine.nmtb.ernwhite (Pbme etrokgg)------ ktzF’@ Nm-way(Pinn!lrvdnnss)______P@ Pitch (Pinns r@id@.__ .. . . . . .._-dii?

—

-—

P@ weatemwhfte (Pfnns montfcola)--iPm, westan yeJlow(Ptnne Pondmoss)-iPfflon&frm9edldf9)..._____J____J

——i%mw=i%i%%!&%!%%%Y_.__J

1%135111103

..—

.

490 REPORT NATIONAL ADVISORY COM3iPl?IlEEFOR AERONAUTICS

small size of the tree or its commercial unimportance.The discussion of the species included will be directedprincipally to the suitability for wing beams, which isthe most important requirement and also the mostdiflicult to meet. However, consideration till alsobe given to suitability for other aircraft parts.

HARDWOOIX3(BROAD-LEAVED SPECIES)

AIthough the data on hardwoods in Table VI arepresented as ratios to spruce, most of the hardwoodspecies have properties that adapt them bettar toconsideration for uses other than wing beams andthe parts commonly made of spruce. The data,however, will serve as well for comparing other speciesamong themselves as for comparison with spruce.The hardwoods as here discussed are considered fromthe standpoint of the aircraft uses to which their prop-erties seem to best adapt them.

RED ALDER (Alnu~ rubra).Red alder is about the same ad spruce in weight and

in most strength properties but exceeds spruce in hard-ness. Although the leading--hardwood of the PacificNorthwest, it isnevertheless a relatively smalI tree thatmatures in 50 to 60 years and reaches at that age adiameter of about 18 inches. The amount of clearlumber available is small.

BLACK ASH (Fraxinua nigra).Black ash is considerably lower in weight than com-

mercial whita ash. It is exceedingly tough and is anexcellent species for bent work. It lacks the strenpthand the sti-ffnessof white ash, however, and can not-beused so satisfactorily where these properties areessential.

BILTMORE WHITE ASH (Fnzxhw Wnoreana).BLUE ASH (F. guadrangulata),GREEN ASH (F. penwyluanica Zanceokzta).WHITE ASH (F. americunu).

Biltmore white, blue, green, and white ash aresimilar in density and mechanical properties and cannot be distinguished from one another by means ofthe wood aIone. They are marketed as white ash,or more properly, as commercial white ash.

Commercial white ash has long been a favorite woodin aircraft where bending and compressive strength,stifkas, shock resistance; and capacity for bendingto a required shape are requisites. It seasons satis-factorily, stays in place well, and presents no manu-facturing M?dculties. It serves as a standard of com-parison where these propertk are essential.

Ash occurs all ovtir the eastern United States andalong the streams in the plains region almost ta thefoothills of the Rocky Mountains. Commercial white

ash lumber is produced from trees of a large range ofsizes, varying from small second-growth timber ta

Iarge virgin trees 100 or more feet in height and 3 or 4feet in diameter.

Northern-grown ash is frequently preferred by thetrade beoause the usual specifications under which ash .iS purchased do not exclude the lightweight, brashmaterial that comes from the swelled butts of swamp-grown trees found in the overflow lands of the lowerMis&sippi ValIey.

0REQ6N ASH (Fratinus oregona).Oregon ash is slight~ylighter than commercial white

ash and slightly lower in its strengti properties. Itsrange is from southern British Columbia throughWashington, Oregon, and California, The Oregonash produced is only a fraction of 1 per cent of the ashconsumed in the United States.

PUMPfiN AsH (Fraxhvw projunda).Pumpkin ash is somewhat. lighter than commercial

white ash and is lower in most of its important strengthproperties. In ash, as in other species, there is a cort-siderable range in the properties of individual piccoaThe term “pumpkin ash” is used commercially to des-ignate the weak, soft material from all species of ash. .SpecMca~one for commercial white ash for aircraf~excIude pumpkin ash,

ASPEN (Populus tremw?oides).Aspen is lighter than spruce and is considcraldg

lower in hardness and stifhss. The tree is quitesmall and the species is nobimportant from the stmul-point of aircraft.

BASSWOOD(Tilia glabra).Basswood is a lightweight wood that is low in

practically dl of its strength properties. It workseasily and can be nailed without splitting, It is avery satisfactory lowdensity spccice for vcncor andplywood and fids other aircraft use.

The range of Tili.a gZabraextends from New Bruns-wick through the Great Lakes region and southward asfar as Pennsylvania and Missouri. Matture trLICS

frequW_tly reach a height of 100 feet and a diameterof 3 to 4 feet,

BEECH (FaguAgrandijolia).Be&ih is quite heavy and has about the strengkh

properties of sweet and yelIow birch and sugar maple.Usually it is not available in the highest grade-s. 1tmight be used to some extent for propellers and ply-wood, but can not be used extensivdy in the frame-work of aircraft.

BeeZh occurs from New Brunswick to northernWNccmsin and south to eastern Texas and westeruFlorida. It reaches its best devolopment in thonorthern States, the lower Ohio ~alley, and thoAppalachians. Mature trees attain a height of 120feet, and diameters of 3 feet or more are not un-common.

AIRCRAJ?I’ TVOODS: TEEEt PROPERTIES, SELE(71!TON, AND CHARACI’ERISTICS 491 —

SWEETBiRCH(Betufa b-da).YELLCIWBHLCH(B. hda).

Sweet arid yellow birch are quite similar in theirproperties and can not be distinguished from eachother by the wood rdone. They are hea-ry, hard,and stM, AU birohes are diihae porous species, havea he, even texture, and are capable of taking ahigh finish. These two species find many uses inaircraft.

The range of sweet birch extends from Newfo@d-hmd to western Ontario, centrt-d Iowa, southernIllinois, south along the Appalachian Mountains,and into F1orida. YeIlow birch occurs from New-foundland ta northern New England, northern Minne-sota, and south along the Appalachians to Tennesseeand North Ctiolina. Mwture trees of sweet birchrange from 70 to 80 feet in height and from 2 ta 3 feetin diameter, while yellow birch is a little larger.

BUTTERX~ (Ju@zne cinema).

Butternut is slightly lower in weight than spruceand is lacking in stiffness, although it is higher inshock resistance. Butternut can perhaps be con-”aidered for wneer and plywood, but it is not likely toattain any importance because of the abundance ofother suitable species.

BLACKCHERRY(Prunua serotina).

B1ack cherry is a moderately heavy wood, eome-what lighter than black wahmt and lower in itsstrength properties. Like black wahmt, it is difluseporous, and is an exceIIent cabinet wood. It is usedfor propellers and shotid, in genersI, be suitable forother parts where walnut is used.

Black cherry occurs from Nova Scotia to the Dako-tas, south to F[orida and &izona, and along the moun-tains to south America. The tree atti”na a height of60 ta 90 feet. Black cherry was once abundant andof considerable cummeroial importance, but thepre&.nt supply is very small. Consequently thespecies can not be expected to be of great importanceto the aircraft industry.

CHESTNUT (Oastanea dentahz).

Chestnut is somewhat heavier than spruoe andexceeds it in hardness, but is lacking in stitheas. Thesupply is fast being depleted because of the ch=tnut ,bhght, so that a oontinued supply can not be expected. ~The wood frequently contains numerous small worm ~holes. Chestnut can be glued satisfactmiIy. Any :ocm=iderable use of chestnut for airoraft is udikely, ‘although it wouId serve satisfactorily for certain ply-wood requirtunents.

The range is from southern Mich@m and the New ~England States southward to F1orida and hlississippi. ~The tree reaches a height of 60 to 100 feet and a diam- ~eter of over 2 feet. i

EAS~EIUTCO~ONWOOD“[POPUIUSdeltoids).NOET%ERN BLACK COTYO=OOD (P. itichocaqnz ha-

tata).BALSAMPOPLAB (P. ba~8ami’fera).

Eastern and northern black cottonwood and balsampoplar rank in strength in the order named. Eastern_cottonwood is abc@ the same weight as spruce aridwith the exception of shock rdst anoe is lower in theother properties listed in TabIe VL Balsam poplaris decidedly lower than spruce in both weight andstrength properties. The wood does not spIit easilyin nailing, however, can be gIued satisfactdy, andbends well. None of these specks w vmlI be con-sidered as a substitute for spruce in wing beams, butalI will perhapa serve some purpose in minor parts.They can aIso be considered for plywood.

Eastern cottonwood is widely distributed through-out the United States east of the Rocky Mountains.Under favorable conditions it grows very rapidly.Mature trees attain a height of 100 feet and over anda diameter of 2 to 5 feet.

Northern black cottonwcmd grows from acuthernAlaska to northern California and eastward as far asIdaho and Nevada. The tree under the best ocmdi-tions attains a height of 80 to 126 feet and a diamet=of 3 to 4 feet.

BaIaam pophr ooours over a vast territmy fromAlaska to Labrador, and southward to Colorado andNew York. h the United States the best timberseIdom exceeds 30 inches in diameter and 60 or 70feet in height, although in the most. favorable sitesoccasional trunks reach a height of 100 feet and adiameter of 6 feet.

AacmmM Em (Ulmus americuna).

American elm, on the average, is lighter than whiteash in weight and is much loyrer in its strength proper-ties. It can be bent to curved form exceedingly welIand its emplopent in aircraft largeIy hinges on theneed for wood having this property. The wood warpsbadIy, however, so that ocmiderable care is necessarytc hold it to form while it is being dried after hgvingbeen bent. It may be used for aircraft plywood.Tery dense pieces of American elm have about thesame properties as rock elm ‘and maybe used whereverrock elm is satisfactory.

American ehn occurs from southern Newfoundlandto the eastern baas of the Rcoky bfountains and southto Texas and F1orida. Commeroitiy it is most impor-tant in Michigan, Wiscmwin, Indiana, and the lowerMississippi Valley. The tree commonly reaches aheight of 100. feet. OccssionalIy it attpins a heightof 120 feet, and a diametw of 6 feet or more.

ROCK ELH (Ulmus racenaosa].Rock elm is slightly heavier than ash, is lower

in stiflness, and &~her in shock reshtance, in which it

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492 REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

excels. It cam be bent to cm-red form readfiy and ifproperly dried after bending can be used in aircraft asan alternate for ash. Considerably more care is neces-sary in the original seasoning and the drying afterbending of rock eIm, however, in order to have itremain in shape, since it twists and warps badIy whennot held firmly.

Rock elm occurs from eastern’ Quebec, throughnorthern New Hampshire and Vermont, to Michigan,WWonsin, northeastern Nebraska, Missouri, andmiddle Tennessee. It is commercially importantchiefly in northeastern Wisconsin and adjscent partsof upper Michigan. The txee occasionally reaches aheight of 100 feet and a diameter of 3 feet.

SLIPPERYELM (lJhnw julva).Slippery elm avw-sges somewhat lighter than ash

and rock elm and is lower in its strength properties.Pieces of a density equal to that of rock elm may beused introchangeably with rock elm. Like the otherelms, it can be bent to curved form readily and sIsotends to warp in seasoning and in drying after havingbeen bent.

Slippery elm has about the same range as kericanelm. It occurs from southern Newfounchnd to SouthDakota and southward to the Ciulf of Mexico. Thetree is somewhat smaller than the American elm andreachea a height of &bout 70 feet.

BLACK GUM (i’?~88aqkatica).Black gum is much heavier than spruce but is lower

in stitlness. Its density is about the same as that ofAmerican elm. The grain is inter~ockedjand althoughthe wood does not split readily, it tenda to warp indrying to a greater extent thm most of the commonhardwoods. It probably will be but little used in theframes of aircraft, although it is acceptable for p~ywood.

Black gum occurs from central New England toFlorida and west to Texas and Michigan. It is awater-loving species. On the best sites it attains aheight of 120 feet md a diameter of 5 feet.

RED GUM (L@w%anabar8tyraci@z).Red gum is heavier than spruce and higher in ita

strength properties, with the exception of stiflness.The grain is interlocked and the wood tends to warpconsiderably. Through careful manufacture and sea-soning, however, material that holds ite shape satis-factorily can be obtained. There is some prospect thatcarefully quarter-sawed red gum, matched for density,may be used for propellers. Studies at the ForestProducts Laboratory have shown that, as far m per-mancy of form is concerned, red gum seems suitable forpropellers if .they are properly manufactured andprotected from extreme humidity conditions. It is acommon veneer and plywood species.

Red gum occurs from southern Connecticut to ~outh-eastern Missouri upd south to Texas AndF1orlda. It ismost abundant in the lower Mississippi Valley. Aver-

age mature trees have a height of 80 to 120 feet and adiameter of 1% to 3 feet. Some mature trees reachdiameters of 5 feet or more.

TUPELOC#u~ (Nys~a aquatica).Tupelo gum is considerably heavier than sprum but

is lower in stiffnms. It seems desirable for aircraftconstruction onIy in the form of plywood.

T-upelo gum occurs aIong the Atlantic and the Gulfcoasts from Viinia to Texas, and northward alongthe Mississippi Valley as far as southern IILinoia, TheIargest trees are about 100 feet in height and 3 to 4feet in diameter above the swelled base. The materialfrom the swelled butt is characteristically light inweight, brash, and weak.

HACKBERRY(CeZti8occidentali8).Hackbarry is lighter than ash, and compmea favor-

ably with ash in shock resistance, although in the othermechanical properties its average values are lower.The denser pieces of hackberry may perhaps be sub-stituted for ash.

Ha&berry occurs from New England to Virginiaand wwtward to North Dakota and Kansas. It is aslender tree, commonly 50 although occasionally 100feet high, and when mature it is from 2 to 3 feet indiameter.

PECAEJHICKORIES.BITTERNUTHICKOR>-(Hicoria cord~onni.e).NUTMEGHICKORY(H. myrMczjormi8),‘iVATEE mcKorm (H. aquatica).PECAN (H, pecan),

The pecan hickories are heavier and more shockresistant than ash and yetj in general, they are inferiorto the true hickories in their mechanical properties,especially in shock resistance.

Their range and size are in general the same as thosoof the true hickories.

TRUE HICKORIES.

BIGLEAF SHAGBARK HICKORY (Uicoria lacinitisa).MOCKERNUT HICKORY (ZZ. alba.)

PIGNUT HICKORY (~. @Zb~U).SHAGBARK HICKORY (H. otxda).

The true hickories are quite simihw in their proper-ties and can not be distinguished from one another bythe wood alone. These hickories are much heavierthan ash, and are very high in their strength properties.The wood is exceedingly tough and in this respectexcels all of the other native species that are commer-oidy available. It is charactmistic of hickory that thecompression fakes in bending do not usually localizebut distribute themselves sc that tho material developsa remarkable toughness. Hickory, being a very densowood, shrinks and swells considerably with change inmoisture content.

Tha true hickories can be substituted in aircraftfor ash but would probably not give quite as good

AIRCRAFT VFOODS: TH3UR PEOPEIB~S, EJIXXOTIOX, AND C=UUCIZIUSTICS 493

service for the same might. Eickov is particularlydesirable where great toughness is required.

The hickories occur over the eastern part of theUnited States, begimirg .m far west as the States onthe west bank of the Mississippi Ri-re.r. The maturetrees am from 100 to 120 feet high and several feet indiameter. Second-growth material or that from smaIItrees is frequently considered superior to that from themature trees, but tests show that the density of thewood, as with other species, is the best criterion ofproperties.

BLACKLocuwc (Bobinia pseudoacacia).Black locust is a ~ery heavy-, strong wood. It

shrinks and sweh but little for its density, havingonly about hdf the shrinkage of hickory for a given

moisture reduction. The heartmood has a high extrac-ti~e content and is =rerydec~y mistant. B1ack locustis an excellent species where hardness and the proper-ties just mentioned are required, but is not likel-yto beof importance in aircraft.

CuouxBm3 MAGNoHA (~Magnoliaacuminakc).EVERGREEN MAGNOLIA (3$. grandi$ora).MomTArN MA~NOu (iM.fraaerio.

The three species of magnolia, on which mechanicaltests have been made, may be considered together.They are alI heavier than spruce, cucumber magnoliaand evergreen magnoIia being considerab~y so. Theyall compare favorably with spruce in their strengthproperties, and conseqwntIy belong to the group ofhardwoods that give promise of being good substitutesfor spruce in wing beams and other aircraft parts,when they are avtiable in proper size and quantity.

The range of cucumber magnolia extends fromwestern New York ta Alabama and westward toIllinois and Mis&sippi. The tree is horn 60 to 90feet high and 2 to 4 feet in diameter.

Evergrem magnolia occurs aIong the coast regionfrom North C?arolina to Florida and westward toTexas, extending through western Louisiana to Arkan-sas. Treea 80 feet high and 4 feet in diameter areoccasionally found.

Mountain magnoIia grows from southern Yiioand northeastern Kentuc@ to northern Georgia andwst and southwest to Tennessee, AIabama, andLcmisiana. The Iarg=t trees are only 30 feet highand 1 foot or more in &meter.

BIGL=F MAPL~ (Acer naacrophylhn].Bigleaf maple is about the same in might as siher

maple and, except in shock titan~~ is mmewhat&mherin strength properties. It is much lighter andlower in mechanimd properitea than the sugar maple.There is probably little use for bighmf maple in air-craft, except possibly for -reneer and plywood.

Bigleaf maple occurs from the coast region of south-eastern Alaska to California. It -iaries greatIy in

&e ~der different growth conditions. Over most _of its range biieaf maple isza small tree averaging “about 50 feet in height and 18 inches in diameter.Under the best conditions it sometimes attains aheight of 100 feet and a diameter of 40 inches or more.

BLACKldkLE (Acer nigmm].%Gm ~~=rt?z (A. ~accharu?n).

B1aek maple end sugar maple, which art.very similarin their properties, are classed and sold as “hard”maple. They can not be &s@@shed from eachother by the wood alone.

These two maphe are dense, hard, and stifl They”are Muse porous, and ha~e a he?. even t=t~e. @account of their hardmw and resistance tu wear theyare often used for the faces of plywood. Thesespecies fid numerous uses in aircraft manufacture.

B1ack maple occurs from Quebec westward tonortheastern South Dakota, and southward ta aboundary from Missouri to West Virginia. Therange of sugar mapIe extends from southern New-foundhnd to Minnesota and southward to easternTexas and northvmstwn ~orida. It is importantcommercially principality in the Lake Stat-, thehTortheast, and the Appalachian region. Maturetrees ordina.dy are from 100 to 120 feet in height,and from 30 to 40 inches in diuneter.

RED MAPLE(A&r rubrum).Red maple is somewhat heavier, stiffer, and stronger

than silver maple but is lower in its mechanicalpropertkis than sugar maple. Red maple couldprobably be used for propellers but the product wouldbe much softer than that made of sugar maple. Itis possible that some of the densest pieces of red maplehave properti= similar to sugar maple and can beused assuch.

Red maple occurs from New Brunswick to Minne-sota and south to Texas and FIorida. The averagesize of mature trees is about 70 feet in height and 2feet in diameter. OccasionaIIy they attain a heightof 100 feet and a diameter of 4 feet.

S.mmmMULE @cer sacchczrinum).SiIver maple, which is much lower in weight and

strength than sugar maple, lacks the stiflne= needed to”make it a satisfaoto~ substitute in aircraft for spruce,and the greater weight is a disadvant~ae. It issuitable for plywood where a semihard speoies isrequired.

Silver maple occurs from New Brunswick to Min-nesota and sauthward to Arkansas and FIorida. Thetrees attain a height of 75 to 120 feet and a diameter of2 to 4 feet.

OBEGONMYRTLE(Uinbellu.hritzcalijornti).Oregon myrtle is much heaviar than spruce tind,

although it is higher in hardness and shock resistance,

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it is decidedly lacking

RJWOET NATIONAL ADVTSORY

in stitlness. It will probablyhave little use in aircraft construction,

Oregon myrtIe oocurs in southwestern Oregon and inCalifornia. In gensral it is not a large tree, but underthe most favorable conditions it growa from 60 to 80feet high and from 2% to 3% feet in diameter.

CALIFORNIA BLACK OAK (Quercw kelloggii).OREGON WHITE OAK’ (Q, gariyamz).ROCKY MOUNTAIN WHITE On (Q. I&&ti).

California black, Oregon white, and Rocky Moun-tain white oak are decidedly lacking in stiflnem andneed not be considered for airoraft service.

COMMERCIALRED OAK,*BLACKOAK(Qzumuawlutimz).LAURELOAK(Q. fuurifolti).

*pm OAK(Q. @fs@).*RED OAK (Q. boredu).*SCARLET OAK(Q. coccin.m).*SOUTHERNREDOAK(Q, rub-a).SwmP RED OAK (Q. rubrapagodzfoliu).WATEROAK(Q. nigra).

*WILLOfi OAK-(Q, phdh).COMMERCIAL WHITE OAK,

*BUR OAK (Quercuamacrocu.rpa).

*CHESTNUT OAK (Q. mon.tana).*POSTOAK (Q. 8te~a).

*SWAB4P CHESTNUT OAK (Q. j9Pi7WS).SWAMP WHITE OAK (Q. bicoZor).

*WHITEOAK(Q: a.lba).There are over a hundred species, varitities, and

hybrids of native oak but most of the oak lumber cutin the United States times from the 11 species in theheading th~t are starred. The various species can bedivided into two broad groups, the red oaks and thewhite oaks, Since the oaks are fairly similar in theirproperties, they will be considered collectively.

Commercial red oak is but slightly lower in weightthan commercial whjte oak and the two groups areunusualIy similar in strength properties, so that differ-entiation need not be made for this reason. Com-mercial red oak, however, is less resistant to decaythan white oak and responds to moisture changesmom rapidly than the whita, The heartwood of thewhite oaks is impervious to liquids, while there isrelatively free longitudinal passage in the red oaks,

The oaks are very heavy and hard and are extremelyvariable in their properties. They need not be con-sidered at aI1 aa a substitute for spruce bub they doplay an important part in propeller manufacture.The radial shrinkage is only about one-ha~f of thatin the tangential direction. Hence quarter-sawedmaterial stays in place much better than plain+awedstock and is more desirable for propeller construction.

The southern oaks, particularly when swampgrown, are very ditlicult to dry properly withoutserious checking, honeycombing, and casehardening.

COMMITTEEFOR AERONAUTICS

They am also reputed to be quite ditIcult to manu-facture, especit-dlyto machine, For this reason therohas been preferenc~ in the trade for upland-grown oak.The available information, however, shows littledifference as far as strength properties am concorncd,

Taken together, the species under consideration arewidely distributed throughout the United States eastof the Great Plains. The size of tho oaks, of courm,varies greatly but at their best they occasionally attaina height of 126 feet and a diameter of 6 feet.

CANYONLrvn C)AK (QuercwschwsoZepie).Canyon live oak is very heavy and is greatly lacking

in stifhwss for ita weight.

LIVE OAK (Querow virginianu).Live oak is one of the densest native species. I?ioccs

of its wood often sink in water, even when air dry,Its shrinkage, however, is no greater than that of theother. oaks, which average much lower in specificgravity. It has remarkable hardness and ti quitotough. Although it can not be considered from thostandpoint of substitution in aircraft for the commonlyused species, ita unuwd charactaristica am worthy ofconsideration in connection with special problems.

Live oak occum along the coast from Virginia tosouthern Florida and to Texas. It is a spreading treaand oonsequentiy yields but short logs. The largestcommercial trees may be about 70 feet high and 6 or 7feet in diameter.

PECM (Hicoria pecan).See the hickories.

PEESIMFJON (Diospyro8 oirginianu).I%rsimmon is a dense wood, very high in hardness,

and has a iine, even texture. It is an excelhmt wood ‘for usee where toughness and smoothness of wear arcrequired, as in shutties, but is probably of no impor-tance in aircraft.

It O-tiUrS from Connecticut to Kansas and south toTexas and Florida. Mature trew are usually littleover 12 inches in diameter but occasionally reach aheight of 100 feet and a diameter of 2 feet.

BALSAIUPOPLAR(Popuhn bal,samijenz).See the cottonwoods.

YEnow POPLU (Liriodimdronfulipijera).Yellow poplar is but little hwvier than spruco and,

although somewhat low in shock-resisting capacity, ithasexcellent working qualities, ability to retain shape,and freedom from checks and shakes. It presents nomanufacturing difficulties. Bccauee of these dasirabloAaracteristics it is regarded as a fairly satisfactorysubstitute for spruce. It is used very extensively forplywobd.

Yellow poplar occurs in all the States east of theMississippi River except Maine, New Hampshire,Vermotii, and Wmonsinj and occurs also in Missouri,

AIRCRAJ?I’TVOODS:TEE~ PROPERTIES,SJZLECI?ION,AND CEURACI’ERISTICS 495

Arkansas, and Louisiana. Yilth sycamore, it is thelargest hardwood tree in the United States. Maturetrees are from 90 to 180 feet in height and from 3 to 8feet or more in diameter.

SUGMtBEERT(Celtie Lmn”gahz).Sugarberry is considerably heavier than spruce but

is lower in stMness. “In properties it resembles hack-berry, to which it is closeIyrelated. As with hackberry,the denser pieces can be substituted for ash.

Sugarberry occurs from Indiana to Missouri andsouthward to Texas and Florida. The tree is notlarge.

SYCA~OEE(Plataww ocoidenfali8).Sycamore is considerably heavier than spruce but,

Iike many of the hardwoods, is lower in stiffness.Shakes are very prevalent. Because of this defectand its low stiffness, sycamore appears to be not suit-able for aircraft, except in veneer and pl~ood whereit is often mixed with red gum, which has approximatelythe same properties.

Sycamore occurs from New England to Nebraska andsouth to Imuiskna and Florida. It is one of the Iargesthardwood trees, attaining a height of 140 feet. Treesseveral feet in diameter are common and some over 10feet have been known.

BLACKWmm (A@zm nigra).B1ack waInut is an excellent furniture and cabinet

wood that has tilreadye&abIished a position in aircraftuse. It probably is the best propeller wood of any ofthe native speoies, since it has good hardness to mistwear and has excellent abtity to retain its shapeunder varying moisture conditions. However, it knotso hard as oak or biroh. Its fine, even texture, excel-lent appearance, and flnishkg qualities make it suitablefor instrument paneIs and cabinetwork. It serves anumber of uses in aircraft but is not considered as asubstitute for spruce.

B1ack walnut occurs from New England to Minne-sota, and south to Texas and FIorida. Forest-grointrees vary in size from a diameter of 2 feet and a heightof 50 feet to a &meter of about 6 feet and a height of100 to 120 feet.

SOllll’WOODS[CONO?EEOuSSPE~ES]

ALASKACEDAB(Cham=cypak nootiafmwis).

Alaska cedm is heavier than spruce, and althoughthe two are about equal in stifbss, it is bigher in theother meohankd propertb. The additional informa-tion available indicates that this cedar is easy to kiln&y, moderately good in ability to stay in phoe, moder-ately e= to glue, and easy to work. The.heartwood ishighly resistant to decay.

‘With its Iimited suppIy, Alaska cedar is not likelyto be considered for use in aircraft from the stand-point of special design. .It should rather be expeoted toserve as a species supplementary to spruc-e, for sub-

dit.ution in spruce sizes; the result is somewhat~eater strength at the expense of increased weight.The range of Alaska cedar is cofied to the Pacfic -”Northwest coast region. The trees not uncommonlyattain a diamet= of 4 to 5 feet and a height of about100 feet.

INCKIJSn CEDAR -(Libocedru.sdecurrens).Incense cedar is somewhat Iighter than spruce,

and is considerably lacking in shock resistance andstiffness. It is not available in huge quantiti= and itsuse in aircraft woold be con6ned w parts that are nothighIy stras.sed.

NORTHEBNWHIfi CEDAR (Thuja occidentals].Northern white cedar is much Lighter than spruce,

and is correspondingly low in all ita strength properties.The heartwood is highly decay resistant. Being acomparatively snd tree with Iittie clear wood,northern white cedar can not be considered as a possi-bility for large or highly stressed parts of aircraftframes.

porLT ORFORD CEDAR (&?na?cypari8 hu?80nia-na).Port Orford cedar is somewhat heavier than spruce,

and equals or exceeds the strength properti= of spruce.Although not an abundant speci-, this cedar can beobtained in large sizes. The wood has a tie appearanceand presents no manufacturing difficulty. The heart-wood is higldy resistmt to decay. Because of therather limited stand and production, the basis of useshould be very aimiIarto that of Alaska cedar; namely,as 8 substitute in aixcraft for spruce in spruce sizes.

Port Orford cedar occurs in the coast region fromsouthwestern Oregon to California, extending inlandabout 40 mike. Matmre trees are usuaIIy from 3% toto 6 feet in diameter, and from 125 to 1S0 feet high;occasional treea are 8 feet in diameter and 200 feethigh.

WESTERNRED CEDAR (Thuja plicata).Western red cedar is considerably lower than spruce

in specifLcgravity and is also lower in strength prop-erties, particularly in stiffness and shock resistance.Although a fairly abundant and durable species, it cannot be used satisfactorily in higldy stressed aircraftparts.

SOUTHKRN C!YPRESS(Taiuna di8fichurn).Southern cypress is somewhat higher in specific

gravity than spruce and, on the average, is equal to orbetter than spruce in strength properties. Cypr=,however, is very high in moisture cxmtent when green,and some of the material is difficult to dry. Theheartwood of mature trees is very durable. Theswelled butts of southern cgpress produce some stocklow in density, which would not be suitable for aircraftuse, but selected mate~al offers a possible spruce sub-stitute. Further study of suitable selection methods

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496 REPORT NATIONAL ADVISORY

should be undertaken, however, before an attempt ismade to me it extensively in aircraft.

Southern cypress occurs in the AtJantic and theGulf ccaata.1regions from southern DeIaware to Texas,and along the bottom lands of the Mississippi and itstributaries to southern Illinois and Indiana. .Maturetrees attain a height of 70 h 150 feet, and a diameterof 4 b 10 feet.

DOUGLASFm (P8eudatsugataxifolia).Douglas h is one of the few species that exhibit a

noticeable difference in properties with region ofgrowth. Material from the Pacific coast, in averagevalues, is considerably higher in weight and strengththan that in the Rocky h[ountain region, and in addi-tion the tree is much larger, affording cIear material inquantity and in large sizes.

Douglas fir from the Pacific coast in general is muchheatier than spruce and its strength properties areequal to or exceed those of spruce. Unlike Sitkaspruce, the density of Douglas fir wood shows a signiii-cant decrease with increase in the height of its positionin the tree, As a result the best aircraft stock wouldcome from the tree at a height of 40 feet or more.Such material would be slightly lower in averagespectic gravity and strength properties than the valuespresented in Table VI, which are averages.

Douglti h is more likely to develop checks in manu-facture and in aervica than spruce. It spIintersacme-what and consequently is more diflicuh to machinethan spruce. The net result is that, aIthough Douglasfir has exceIIent strength properties, its other charac-tarieticsare such that considerable more care in manu-facture is required than with spruce. Douglas fir isnot likely to be used extensively for aircraft wingbeams as long as spruce is awdable; it is used, how-ever, for veneer and plywood,

AIthough Douglas fir grows over a hrge area inthe West, the so-called Pacific coast type just discussedis cotined to the coast region from southern Oregon toBritish Columbia, It is one of the hrgest commercialtrees of the world, being from 3 to 10 feet in diameterand 176 to 300 feet in height.

Douglas fir from the Rocky Mountain region issomewhat heavier than spruce. It is generally lowerin shock resistance, can not be obtained in large sizes,is decidedly knotty, a~d hence is not considered asatisfactory substitute for spruce.

In Idaho and the region immediately arohd it,however, some Douglas h that is satisfactory for air-craft stock can be obtained,

ALPINEFm (Abie8 .hz8ioc.ar-pa).Alpine flr is very low in might and also in all its

strength properties, TThether some stands containmWwial that is appreciably stronger than the speci-mens tested is not known, but this species appeam to

COMMITTEEFOR AERONAUTICS

hold no promise of furnishing stock for highly stressedaircraft parts.,

BALSAMI?m (Afn2aba.lsamea).Balsam fir is lighter than spruce and, is lower in its

strength properties, being particularly deficient inhardness and in shock resistance. It gives littlopromise of being a satisfactory aircraft wood.

CALIFORNIARED FIR (Abtis mugni~).California red fir is the same in average weight as

spruce, and is very similar in iw mechanical proportics.Little is known of its other characteristics, such as easeof drying, workabdity, and ease of manufacture.These factors must be in~estigated before it can berecommended for aircraft service, and proper methodsof selection must also be developed.

California red fi grows in California and southmnOregon. It is commonly from 125 to 175 feet high andfrom 30 to 50 inches in diameter,

LOWLANDWHITE FIR (Abies grandi8).SILVER FIR (.A. amubilti).T7HITE FIR (~. condor).

Lowland white, silver, and white fir promise tofurnish but Iittle clear material in sizes thmtare suit-able for aircraft. They are of interest chiefly becausethey may be used for veneer and plywood.

NOBLEFIR (Abh nobilis).Noble fir is slightly lighter in weight than spruce,

and compares favorably with spruce in its strengthproperties From the strength standpoint, therefore,noble fir is a possible spruce substitute in aircrRft.Mature trees are said b yield a high percentage ofclear stock. Its use seems to hinge on ita othercharacteristics, concerning which additiomd informa-tion is needed. These characteristics include methodsof drying and handling, methods of selection, tendencyfor checks ta develop, workability, ease of drying,and facility of manufacture.

Noble fir is found in Washington, Oregon, andCalifornia. Large trees are from 140 to 200 feet inheight, and from 30 to 60 inches in diameter.

EMTER~ HEMLOCK(Tsugczcunadmsis).Eastern hemlock compares favdrab~y in weight and

in strength properties with Sitka spruce, buLbecauseof its other characteristics it need not be consideredfor aircraft, ahthough it is a much used lumber andtimber species. The amount of clear lumber in largesizes is relatively small.

MouN@N HEMLOCK(Zwqa nwrfmsi.ana).The average weight of mountain hemlock is much

greater than that of spruce. . Although this speciesexceeds spruce in most of its strength propcrtlcs, itcan not be regarded of importance as an aircraft

AIRCRAFT ‘iVOODS:TEEIR PROPERMS, SEGECTI027,AND ~Ct53RISTICS 497

wood because of its Iimited supply and the inacces-sibility of its stands.

WESTERNHEMLOCIC(Tsuga heterophylla).Western hendocli is slightly heavier than spruce,

and equals or exceeds that species in its strengthproperties, although it is somewhat less uniform intexture than spruce. Studies at the Forest ProductsLaboratory show that it can be Mn dried and ghuxlsatisfactorily-. Western hendock is an abundantspecies. The trees, however, me not so large asspruce and have a lower percentage of clear material,and hence the percentage of material that wilI meetakraft requirements is smaller than that of spruce.Yet, all things considered, it should be regarded asa possible substitute in aircraft for spruce in sprucesizes. It is now used for -ieneer and plywood.

Western hendock grows from northwestern Mon-tana and northern Idaho to Alaska and aIong thecoast ranges and the Cascades to California. Underfavorable growth conditions the trees reach a heightof 200 feet and a diameter of 3 or 4 feet. Largertrees are found, but those over 5 feet in diameter arerum.

~ESTERN LARCH (bfi 0CCidt71taliS).

The average weight of western Iarch exceeds that ofspruce, and the wood is aIso higher in its strengthproperties. The material horn the Iower portion ofthe trees contains a gum crtlIedgalactan, which addsgreatly to the weight. Four to eight feet of the lowerportion of the tree is often discarded because of weightand the prevalence of shakes. Western Iarch, likesome of the dense softwoods, exhibits a Iarge variationin properties, and there is etidenoe of ootiderabledifference in materhd from difterent sites. AIthoughthis larch is high in strength properties, it seems notfeasible to use the species for aircraft, in tiew of thesupply of more suitable woods.

JACK PI~ (Pinw banktina).Jack pine is slightiy heavier than spruce, and is

especially lacking in stithss. The tree is relativelysmall and does not appear to be a promising aircraftmaterial.JEFFREYPINE (Pinus jqfreyi).

Jefiey pine is about the same w&”ht as spruce butit is lacking in stiftness and shook resistance. It is notan abundant species and shows no especial suitabilityfor aircraft.LODGEPOLE PIIQZ (l%zw cmdorta).

Lodgepole pine is very similar @ weight to sprucebut is slightly lower in most of its properties and isparticularly lacking in shock resistance. OnIy about20 per cent of the trees are kuge enough for saw timberand the abundance of small knots makes it dMcuIt toobtain much clear stock. Consequently it is not likelythat lodgepoIe pine will be used for aircraft.

~OBTHERN W=E P~E (~im.fs .stroha).

h’orthern white pine is somewhat lighter than spruceand Iovmr in its strength properties, particularly inhardness and shock resistance. It has an enviablereputation as a wood that is uniform in properties,stays in place welI, presents no manufacturing difE-culties, seasons easily, and maybe glued satisfactorily.Northern white pine is recommended for aircraft serv-ice; although it couId perhaps be used in spruce sizesbetter practice would probably be to redesign becauseof the slightIy lower design vabs of strength. Witha little addit.ionaIcam in SeIection, however, it couldperhaps be used in spruce sizes.

The rmge of northern whit-e pine extends fromCanada into the Great Lakes States and the New Eng-land States and south “in the Appalachians as far asnorthern Georgia. The virgin timber is usualIy 3 feetor more in diwneter and attains a height of 100 feetor more. htensive logging has made heavy irwoadsonthe virgin supply of this wood, however, and the whitepine bIister rust threatena its future growth. Thesupply of suitable stock is not huge enough to developthe use to tie extent that its ddrable propertiwwouId otherwise permit. .

NORWAY PINE (Pinua resin.xa).

Norway pine is considerably heavier than spruceand is higher in its strength propertk. The wood ismore resinous than that of the white pines and thesummer wood is more pronounced. Experiments showthat it can be IriIndried satisfactofiy. Although Nor-way pine can be safely used in aircraft in spruce aimsat the expense of the additional weight that will thenresult, its considerably higher properties indicate thqtbest resuhs would be obtained from special desigg.

Its range is from New Brunswick to Manitoba andsouth to Minnesota and West Viia. The treereaches a height of 90 feet or more and a diameter ofabmt 3 feet.

SOUTHERN YELLOW l?rmzs.LOBLOLLY P~E (%am taeda).LONGLEAF P~E (~. @z@-is).

h~OUNTAIN PINE (P. pungens).PD7CH PINE (P. ri@?a).PONDPINEI (P. ri@h 8t3rO#i7UZ).

SAND PINE (P. C.kmm).~HOETLE= PINE (P. echinata).-H PDSE (P. czzriba).

The southern yellow pines, in gened, are =trOmelyresinous, with a pronounced summer wood that is verydense. The southern yelIow pines as a whole are from36 to 70 per cent heavier thlm spruce and are muchhigher than spruce in strength; Table VI presents adetaiIed comparison. Although they have exce~entproperties, they are not so strong for their weight assome of the other pines.

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498 REPORT NATIONAL ADVISORYCO~ FOR AERONAUTICS

Individual pieces of any of the southarn yellow pinespecies exhibit a large range in density and properties.Any material for aircraft wing beams from theaauthernyellow pines is most likely to come from the Iighterweight, slow-growth stock. However, even this lighterweight material, such as that represented in the tradeby “Arkansas soft pine,” would have difficulty incompeting with the other..specias that are possiblesubstitutes.

The southern yellow pines grow in the Atlantic coastand the Gulf coast regions from New Jersey to Texas,and in the lower MississippiRiver drainage areu. Theyattain a height of 50 to over 100 feet and a diameterof 15 to 48 inches.

SUGARPINE (Pinm lambertiuna).Sugar pine is lighter than spruce and is lower in

its strength properties, particularly shock resistanceand stitlness. It can be worked satisfactorily andstays in place quite well, approaching to some extantthe desirable characteristics of northern whita pine.Sugar pine is not so easy to season as spruce or whitepine. It htis a ‘relatively high moisture content whengreen and consequently attempts h hastan seasoningby high temperatures must be avoided. The strengthproperties are too low to permit its use in aircraft inspruce sizes, but the species does offer a possibilityfor use through special design.

Sugar pine occ~ in the mountain regions of southernOregon and California. It is most abundant andreaches its best development in the Sierras. The treesattain large sizes, heights of 200 feet or more anddiameters of 6 feet beii common.

WESTERNWHITE PINm (Pinue moniicota).Western white pine is about the same weight as

spruce, but is slightly lower in shock resistance andmuch lower in hardnwa. Experiments at the ForestProducts Laboratory show that western white p“inecan be kiln dried without damage b the strengthproperties. It presents no particular manufacturingdif%cuhies and stays in place quite satisfactorily.This species could probably be substituted in aircraftfor spruce in spruce sizes but, as with northern whitapine, better practice would be to redesign.

The range extends from southern British Cohnnbiato weetern Montana and couth along the Caecadea andSierras to central California; the “region of greatestimportance is the Panhandle of Idaho and the adjacentparts of Washington and Montana. Mature trees ofwestern white pine frequently reach heights of 100 toMO feet and diameters of 5 feet or more.

WESTERNYELLOWPINE (Pinw pmderosa).

A1though western yelIow pine is dightly heavierthan spruce, it is also lower in its strength properties,being particularly deficient in shock resistance and instifhss. The wood is more variable than that of the

white pines and in consequence visual methods ofse.kwtionare perhaps somewhat IW reliable than formany of the other woods. Although it presents noawious manufacturing difficulties, and can bc gluedsatisfactorily, its variability and inclination tu l.mash-ness make its use as a spruce substitute in aircraft veryqueetionable unless the use is based on accoptnncc t.cste,such as the taughness-test method,

Western yellow pine occurs frcm British Columbiaand the Black HilIa southward in tho Pacific and RockyMountain region tc western Texas and hfexico. Thetiee attains a height of 100 to 300 feet and a diameterof 6 feet, and occasionally more.

REDWOOD (fle~”a sempervirtm8).Redwood is considerably heavier than spruce and

is about equaI to or exceeds spruce in ita strengthproperties. Although redwood shrinks and swells butlittle with change in moisture, it ia dficult to season,particularly the material from the lower part of thetree. Redwood is somewhat more variable than manyof the other species. The depth of the sapwood isrelatively small. The heartwood contains compound8soluble iu odd water, which are eallod extractives, andwhich may add as much as 12 or 16 per cent to theweight but do not increase all the strength propertiesas a like amount of wood substance would increasethem. This deficiency is particularly .apparent inshock resistance. Redwood, in an unseasoned con-dition, exhibits strength properties (except shockresistance) that are generaI1yhigh for ita weight, but1- increase in strength results from seasoning thanis normal for most woods. The heartwood is varydeoay resistant. Because of variability, drying diill-cultk, and the likelihood of obtaining brash maimialtit appears that redwood is not a desirable sprucesubstitute in aircraft.

The range of redwood is very limited, being con-fined principally to Humboldt, Mendocino, and DelNorte Counties of California, Average mature treesare from 200 to 300 feet in height and from 8 to 12 feetin diameter. Much larger trees are nob uncommon.

BLACKSPRUCM(Pi.ceamarimyz).B1ack spruce is slightly heavier than Sitka spruce,

but @ similar in strength. The trees attain a heightof 40 to 80 feet and a diameter of 1 to 2 feet. Thesupply is not large and, since the quantity of clear stockin suitable sizes is small, the species is nob of enoughimportance commercially to warrant oonaiderationfor airoraft use.

EN~ELMANNSPRUCE(Picea cngelmannii).Engelmann spruce, as found in the Rocky Mountain

region of the United States, is much lighter than Sitkaspruce and is very decidedly lower in its eevmdstrength properties. This material, if considered atall for airoraft, would necessitate redesign, which would

AIRCRAI?I’ WOODS: TEEIR PROPER~S. SELECTION, AXD ~4C71ERISTICS 499

involve beam sizes considerably hwger than thoserequired for Sitka spruce. There is evidence, however,that Engehmmn spruce is one of the species that showa di&erence in properties for different parts of theirranges. ldaterid from the more favorable sites inBritish Columbia appears to be much denser and higherin strength than that from the Iovrer Rocky Xfountainregion, and appears to be very similar to Sitlia sprucein these properties. A the necessity for substitutesfor Sitka spruce in aircraft develops, special considera-tion can well be given Engelnmnn spruce from themore favorable part of its ~~e and studies shouldbe made to appraise its suitability more definitely.

The range is from ~ulson and British Columbia tosauthern Oregon and through the Rocky hlountainsinto New Mexico and Arizona. The tree is usuallyfrom 60 to 100 feet in height, although at high altitud~it may resemble a mere shrub. The diameter rangesup to about 36 inches but in most cases it is much Iws.

RED SPRGCE(Picea rubra).SITKA SPRUCE (P. ~“tchenais).TRITE SPRUCE (P. glauca),

Although ‘these three spruces are simihr in theirproperties and may be used interchangeably for air-craft parts, Sitka spruce, because of its Iarge sizelavaiIabiIity, and proportion of clear stock in suitablesizes, is far more important than red or white and isthe chief source of supply.

Red, Sitka, and white spruce possess excellentstrength properties, with a high ratio of strength toweight. They are relatively easy to season, can beglued with fadity, and present no manufacturing dMi-cuIties. As a re@t they serve very satisfactorily forthe higldy stressed parts of aircraft frames and findextensive use in wing beams. They are consideredthe standard of comparison for suitability.

Red spruce is found in eastern Canada and the east-ern United Statea from hTevrBrunswick as far south asNorth CaroIina. It reaches a height of 70 to 100 feetand a diameter of 2 to 3 feet.

Sitka spruce occurs in a strip rdong the Paciticcoast from northern California to Ah&a and usually isnot found more than 40 miles inland. The trees areurdinardy from 80 to 125 feet in height and 3 to 6 feetin diameter. Larger trees, which are not uncommon,reach a height of 180 feet and a diameter of 12 feet.

White spruce grows from Alaska to Quebec andsouthward to Montana and West Via. Thehwgest trees are 100 feet in height and 3 feet in diam-eter, but most white spruce trees are smaller.

TAMARACK(Lmix ktitina).

Tamarack is much heavier than spruce, and ishigher in strength. This species would probably fur-

41030-31~

nish little clear material and need not be considered foraircraft use.

CONCLUSIONS

The chief merits of wood for aircraft constructionare a I@h ratio of strength to might; an inherentlightness in might, which for a givcm depth of memberpermits considerable width to afford lateral stabilityagainst buckling; the ease with which it can be manu-factured and assembkd, and for the same reason,the ease with which it may “be repaired withoutspeciaI equipment; its relative cheapnws; the adapta-bility to both Iarge-scale and smakxde production;the ease with which it may be glued and spIiced; andthe adaptability to close design, with which is includedperfect freedom in the determination of sizes of parts,without the handicap of manufacturing clifhdties orstandardized sizes.

The studies at the Forest Products Laboratory ofstandard shapes, effect of form on strength and stifhess,stability of thin, outstanding flanges+ factors affecti~~strength, properties of different species, and methods ofedection have yieIded extensive and rather compIeteinformation covering the use of wood in aircraft.The detaiIed knowledge of these factors has placed air-plane design with wood on an excellent basis ofreliability.

Although relatively few species are now used inaircraft production, there are in addition a very consid-erable number from which further choice may bemade. In generrd, such additional species fall intothree classess: (1) Species that have properties andcharacteristics similar to woods now used and thatmay be substituted directly for them (such direct sub-stitution may involve, in some cases, an increase inthe minimum specific gravity limitation over thatgiven in Table I); (2) species that maybe consideredon the basis of special design; (3) species that seemsuitable from the standpoint of strength, but on whichadditional information relating to selection and manu-facture are desired. Chief among the species that maybe considered along with red, Sitka, and white sprucefor highly stressed parts, such as wing beams, areAlaska cedar, Port Orford cedar, southern cypress,Douglas h, noble fi, western hemlock, northernwhite pine, Norway pine, sugar pine, western whitepine, yellow popkr, and Engehmmn spruce.

FOREST PRODUCTS LABORATORY,FOREST SERVICE, UNITED STATES

DEPARTIJENT OF AGRICULTURE,

hhmso~, ‘iVrs., April 1, 1930.

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REFERENCES

Reference 1. American Society for Teatiig” Materials: Stand-

ard Methods of Testing Small Clear Specimens of Timber.

A. S. T. M., Serial Designation D143-27, 1027.

Reference 2. Hardecker, John F.: Application and Fabrica-

tion of Wood in Airplanes. National Lumberman, June,

1929.

Reference 3. Markvmrdt, L. J.: Compwative Strength prop-

erties of Woode Grmvn in the United Statm. U. S, Dept.Agr. Tech. Bu1. 158, 1930.

Fteference 4. Mathew80n, J. S.: The Air Seasoning of ‘Wood.U.S. Dept. Agr. Tech. BUI. 174, 1930.

Reference 5. Newlin, J. A. and !l?rayer, G. R’.: De.tlectJon ofBeams with Special Reference to Shear Deformations.N. A. C. A. kpOrt No. 130, 1924.

Referenoe 6. Nevdin, J. A. and Trayer, G. W.: Form Factorsof Beams Subjected to Transverse Loading Only. N. A. C.A. Report No. 181, 1924.

Reference 7. Snyder, Thomas E.: Defects in Timber Causedby Insects. U.S. Dept. Agr. Bu1. 1490, 1927.

Reference 8. Sudworth, George B.: Check List of the ForestTrees of the United States, Their Namex and Rangw. U. S.Dept. Agr, Misc. Cir. 92, 1927.

Reference 9. ‘iVatkins, J. R.: Pitch Pockets and Their Relationto the Inspection of Airplane Parte. Journal of the FrankIinInstitute, August, 1919.

Reference 10. Nllcon, T. R. C.: The Effect of Kiln Drying onthe Strength of Airplane Woods. N. A. C. A. Report No.68, 1920.

Reference 11. Wilson, T. R. C.: The Effect of Spiral Grain onthe Strength of Wood. JouruaI of Forestry, November, 1921.

BIBLIOGRAPHY

Allen, S. W., and Truax, ‘I’.R.: Glu~ Used in Airplane Parts.N. A. C. A. Report No. 66, 1920.

Boyce, J. S.: Decays and Discolorations in Airplane TVoods.

U. L% Dept. Agr. Bul. 1128, 1923.

EImendorf, Armin: Data on the Design of Plywood for Air-craft. N. A. C. A. Report No. 84, 1920.

Forest Products Laboratory, Forest Service, United StatesDepartment of Agriculture: Manual for the Inepcction ofAircraft Wood and Glue for the United States Navy. NavyDe@ment., Bureau of Aeronautics, 1928.

Newlin, J. A., and Wiion, T. R. C.: Mechanical Properties ofWoods Grown in the United States. U. S. Dept. Agr. Bul.556, 1917.

Newlin, J. A., and Trayer, G. W.: Stresses in Wood MembersSubjected to Combined Column and Beam Action. N. A.C. A. “Report No. 188, 1924.

Paul, B. H.: The Application of Silviculture in Controllingthe Specific Gravity of Wood. U. S. Dept. Agr. Tech. Bul.168, 1930.

Sparhai’k, W. N.: Supplies and Production of Aircraft Woods.N. A. C. A. Report No. 67, 1919.

Thelen, Rolf: Kiln Drying Handbook. U. S. Ikpt. Am. Bul..-

1136, 1929,Truax, T. IL: The Gluing of Wood. U. & D;pt. Agr. Bul.

1600, 1929.